Comment # 8 on bug 1087525 from Takashi Iwai

Damn, it misses that option yet.  I need to patch and send to upstream...

OK, then try to blacklist this module at first.  Add the following line to
/etc/modprobe.d/99-local.conf

  blacklist amdgpu

And run mkinitrd to rebuild initrd, and reboot without "nomodeset" option, and
without "quiet" option, but with "3" option.  This will avoid starting X.

Then try to load amdgpu driver:
  modprobe drm debug=0x0e
  modprobe amdgpu

Also, try a newer kernel, e.g. the kernel from OBS Kernel:stable repo.
If this works with amdgpu driver, we might be able to backport something.