I cloned "~agd5f/linux" on the branch "drm-next-4.14-wip" and copied my .config
from the 4.12.4 I was using.
`uname -r` give me "4.12.0-rc7+"

Now `modprobe amdgpu` is freezing my screen instead of blackscreen.
`dmesg | grep amdgpu -i` give this :

[  107.804074] [drm] amdgpu kernel modesetting enabled.
[  107.804109] fb: switching to amdgpudrmfb from EFI VGA
[  107.804672] amdgpu 0000:01:00.0: SI support provided by radeon.
[  107.804674] amdgpu 0000:01:00.0: Use radeon.si_support=0 amdgpu.si_support=1
to override.

Using 4.12.4 my second screen was fine, that isn't the case anymore with
drm-next-4.14-wip.
Using 4.12.4 I could startx (with a second screen obviously, my main screen was
remaining black) now startx seems to return "no screen found".

Note that radeon is still working perfectly.