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: NTom St Denis <tom.stdenis@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Acked-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>