Documentation/amdgpu_dm: Add DM color correction documentation

AMDGPU DM maps DRM color management properties (degamma, ctm and gamma)
to DC color correction entities. Part of this mapping is already
documented as code comments and can be converted as kernel docs.

v2:
- rebase to amd-staging-drm-next
- fix typos (Tales)
- undo kernel-docs inside functions (Tales)

Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>