Raven ridge APU (2400g), no external GPU, linux-4.17-rc2, libdrm-2.4.91,
mesa-18.1.0-rc1, xorg-server-1.19.6, xf86-video-amdgpu-18.0.1.

So I've tried to reproduce https://bugs.freedesktop.org/show_bug.cgi?id=106276
using simple script:
```
#!/bin/sh
set -ex
while true; do
    set -x
    X&
    XPID=$!
    sleep 1
    kill ${XPID}
    sleep 1
done
```

and ended up with screen showing nothing (i. e. backlight enabled, but all
image is black) in both console & X. dmesg is full of
[Fri Apr 27 16:17:33 2018] [drm:dc_add_plane_to_context [amdgpu]] *ERROR*
Surface: can not attach plane_state 00000000a97ad24a! Maximum is: 4
[Fri Apr 27 16:17:33 2018] [drm:dc_add_plane_to_context [amdgpu]] *ERROR*
Surface: can not attach plane_state 00000000d38d9362! Maximum is: 4
[Fri Apr 27 16:17:33 2018] [drm:dc_add_plane_to_context [amdgpu]] *ERROR*
Surface: can not attach plane_state 00000000d38d9362! Maximum is: 4
[Fri Apr 27 16:17:33 2018] [drm:dc_add_plane_to_context [amdgpu]] *ERROR*
Surface: can not attach plane_state 00000000d38d9362! Maximum is: 4
[Fri Apr 27 16:18:01 2018] [drm:dc_add_plane_to_context [amdgpu]] *ERROR*
Surface: can not attach plane_state 00000000b7ff318d! Maximum is: 4
[Fri Apr 27 16:18:04 2018] [drm:dc_add_plane_to_context [amdgpu]] *ERROR*
Surface: can not attach plane_state 00000000b7ff318d! Maximum is: 4