drm/amd/display: add comments to describe DM crtc color mgmt behavior

Describe some expected behavior of the AMD DM color mgmt programming.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>