drm/amd/amdgpu: Port GMC v6 driver to new SI headers (v2)

Port the GMC v6 driver over to the new SI headers.

Tested with a Tahiti SI ASIC.

(v2) Fixed a couple of typos (in commented code) and moved
     defines to si_enums.h

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