I have merged
https://cgit.freedesktop.org/~agd5f/linux/log/?h=drm-next-4.10-wip into 4.9 and
am now using amdgpu with the r9 390, and everything works great so far except
that the cursor is no longer visible after hibernating and resuming, but
switching to SWCursor resolves this, so this is a viable workaround. I will
open another bug for this, but it is only a minor issue for me. With radeon,
DPM still fails to resume properly for the 390 after hibernation.