drm/amd/amdgpu: port of DCE v6 to new headers (v3)

drm/amd/amdgpu: port of DCE v6 to new headers (v3)

Port of SI DCE v6 over to new AMDGPU headers.  Tested on a
Tahiti with GNOME through various hot plugs/rotations/sizes/fullscreen/windowed and
staging drm/xf86-video-amdgpu.

(v2) Re-factored to remove formatting changes to si_enums.h
     as well rename various defines.
(v3) Rebase on upstream

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>