Any chance you can use git bisect to find the change between 4.8 and 4.9 which
introduced the problem?

(In reply to Reimar Imhof from comment #6)
> Do I need a special config for loading the amdgpu driver?
> 
> There is no video-driver specific config in /etc/X11/xorg.conf.d/

There should be a /usr/share/X11/xorg.conf.d/10-amdgpu.conf file which causes
the amdgpu driver to be used automatically for GPUs controlled by the amdgpu
kernel driver. If that file is missing, please report the problem to openSuse.