I found this:
viktor@marfa:~]$ tail -1000f /var/log/X.0.log
[ 5.992]
X.Org X Server 1.20.5
X Protocol Version 11, Revision 0
[ 5.992] Build Operating System: Linux 4.19.92 x86_64
[ 5.992] Current Operating System: Linux marfa 4.19.107 #1-NixOS SMP Fri Feb 28 15:39:01 UTC 2020 x86_64
[ 5.992] Kernel command line: initrd=\efi\nixos\kw2vwvhnq89mzinrg5xrwjvr1vxzx5xm-initrd-linux-4.19.107-initrd.efi systemConfig=/nix/store/m07ghssizc5agi36wsn0vvhrqp2ica32-nixos-system-marfa-19.09.2201.7d31bbceaa1 init=/nix/store/m07ghssizc5agi36wsn0vvhrqp2ica32-nixos-system-marfa-19.09.2201.7d31bbceaa1/init loglevel=4
[ 5.992] Build Date: 30 May 2019 06:28:09PM
[ 5.992]
[ 5.992] Current version of pixman: 0.38.4
[ 5.992] Before reporting problems, check http://wiki.x.org
to make sure that you have the latest version.
[ 5.992] Markers: (--) probed, (**) from config file, (==) default setting,
(++) from command line, (!!) notice, (II) informational,
(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
[ 5.992] (++) Log file: "/var/log/X.0.log", Time: Mon Mar 16 17:07:27 2020
[ 5.992] (++) Using config file: "/nix/store/rfbmghnc776jz0n95vv0b06dw6in4cqg-xserver.conf"
[ 5.992] (==) Using config directory: "/etc/X11/xorg.conf.d"
[ 5.992] (==) Using system config directory "/nix/store/az5r35q73m4rav5kh2bppvdxn86lf8rv-xorg-server-1.20.5/share/X11/xorg.conf.d"
[ 5.992] (==) ServerLayout "Layout[all]"
[ 5.992] (**) |-->Screen "Screen-amdgpu[0]" (0)
[ 5.992] (**) | |-->Monitor "<default monitor>"
[ 5.993] (**) | |-->Device "Device-amdgpu[0]"
[ 5.993] (==) No monitor specified for screen "Screen-amdgpu[0]".
Using a default monitor configuration.
[ 5.993] (**) Option "DontZap" "on"
[ 5.993] (**) Option "AllowMouseOpenFail" "on"
[ 5.993] (==) Automatically adding devices
[ 5.993] (==) Automatically enabling devices
[ 5.993] (==) Automatically adding GPU devices
[ 5.993] (==) Max clients allowed: 256, resource mask: 0x1fffff
[ 5.993] (**) FontPath set to:
/nix/store/agmg2bsd0psajwjgljnxsp26fk3sh523-font-bh-lucidatypewriter-100dpi-1.0.3/lib/X11/fonts/100dpi,
/nix/store/w3vf2j0x28973jp7frc75d1zh6nl9kw2-font-bh-lucidatypewriter-75dpi-1.0.3/lib/X11/fonts/75dpi,
/nix/store/6y2sndvjjiivsdrn5js8sxdjgqmhhvcw-font-bh-100dpi-1.0.3/lib/X11/fonts/100dpi,
/nix/store/zc93fsfsa9l5vsc6hn7cmsws6x75hcgc-font-misc-misc-1.1.2/lib/X11/fonts/misc,
/nix/store/3qgmkvj45pga7kcrxzbjfpp9sxw6hw4x-font-cursor-misc-1.0.3/lib/X11/fonts/misc,
/nix/store/nyx1r5a0vnw0s84p8nh1mqcln89rm3rl-unifont-12.1.03/share/fonts,
/nix/store/whz1bqyywxydwikqf8c4caglx2ci49lq-font-adobe-100dpi-1.0.3/lib/X11/fonts/100dpi,
/nix/store/w4yy0x0cwmh78gkzph87zmm50bmnh8ba-font-adobe-75dpi-1.0.3/lib/X11/fonts/75dpi
[ 5.993] (**) ModulePath set to "/nix/store/cnxxas612fqrkb04wkqhbhq6nkycdpwy-xf86-video-amdgpu-19.0.1/lib/xorg/modules/drivers,/nix/store/az5r35q73m4rav5kh2bppvdxn86lf8rv-xorg-server-1.20.5/lib/xorg/modules,/nix/store/az5r35q73m4rav5kh2bppvdxn86lf8rv-xorg-server-1.20.5/lib/xorg/modules/extensions,/nix/store/az5r35q73m4rav5kh2bppvdxn86lf8rv-xorg-server-1.20.5/lib/xorg/modules/drivers,/nix/store/6hd0q9rh3wwgmb3r1nv8w28bw1mlhayr-xf86-input-evdev-2.10.6/lib/xorg/modules/input,/nix/store/r9ck6a4i1aw0b1phwz6v73pgdylw5a9l-xf86-input-libinput-0.28.2/lib/xorg/modules/input"
[ 5.993] (II) The server relies on udev to provide the list of input devices.
If no devices become available, reconfigure udev or disable AutoAddDevices.
[ 5.993] (II) Loader magic: 0x626d40
[ 5.993] (II) Module ABI versions:
[ 5.993] X.Org ANSI C Emulation: 0.4
[ 5.993] X.Org Video Driver: 24.0
[ 5.993] X.Org XInput driver : 24.1
[ 5.993] X.Org Server Extension : 10.0
[ 5.993] (++) using VT number 7
[ 5.993] (II) systemd-logind: logind integration requires -keeptty and -keeptty was not provided, disabling logind integration
[ 5.994] (--) PCI:*(3@0:0:0) 1002:731f:1458:2313 rev 193, Mem @ 0x70000000/268435456, 0x80000000/2097152, 0x9a200000/524288, I/O @ 0x00004000/256, BIOS @ 0x????????/131072
[ 5.994] (WW) Open ACPI failed (/var/run/acpid.socket) (No such file or directory)
[ 5.994] (II) "glx" will be loaded by default.
[ 5.994] (II) LoadModule: "glx"
[ 5.994] (II) Loading /nix/store/az5r35q73m4rav5kh2bppvdxn86lf8rv-xorg-server-1.20.5/lib/xorg/modules/extensions/libglx.so
[ 5.995] (II) Module glx: vendor="X.Org Foundation"
[ 5.995] compiled for 1.20.5, module version = 1.0.0
[ 5.995] ABI class: X.Org Server Extension, version 10.0
[ 5.995] (II) LoadModule: "amdgpu"
[ 5.995] (II) Loading /nix/store/cnxxas612fqrkb04wkqhbhq6nkycdpwy-xf86-video-amdgpu-19.0.1/lib/xorg/modules/drivers/amdgpu_drv.so
[ 5.995] (II) Module amdgpu: vendor="X.Org Foundation"
[ 5.995] compiled for 1.20.5, module version = 19.0.1
[ 5.995] Module class: X.Org Video Driver
[ 5.995] ABI class: X.Org Video Driver, version 24.0
[ 5.995] (II) AMDGPU: Driver for AMD Radeon:
All GPUs supported by the amdgpu kernel driver
[ 5.995] (II) AMDGPU(0): [KMS] drm report modesetting isn't supported.
[ 5.995] (EE) Screen 0 deleted because of no matching config section.
[ 5.995] (II) UnloadModule: "amdgpu"
[ 5.995] (EE) Device(s) detected, but none match those in the config file.
[ 5.995] (EE)
Fatal server error:
[ 5.995] (EE) no screens found(EE)
[ 5.995] (EE)
Please consult the The X.Org Foundation support
at http://wiki.x.org
for help.
[ 5.995] (EE) Please also check the log file at "/var/log/X.0.log" for additional information.
[ 5.995] (EE)
[ 5.996] (EE) Server terminated with error (1). Closing log file.
Also found a newer issue here:
Hm…