Comment # 7 on bug 1188954 from Stefan Dirsch

Ok. If it fails only during initial X startup, this looks like a timing issue,
i.e. kernel module is not being initialized in time before X gets started.
Maybe amdgpu kernel module is missing from initrd, but radeon is (since it's
the default driver), i.e. adding amdgpu to initrd may help (if it's really
missing). I can't say anything about other Linux distros. They may use
completely different kernel versions and patches for them.
On openSUSE amdgpu DDX is being used when "amdgpu" kernel module is being
loaded. There should be no need to configure it when the package
xf86-video-amdgpu is being installed. I still don't understand why you want to
use "amdgpu" drvier with your hardware though.