drm/amd/display: Add comments.

Reviewed-by: Dennis Chan <dennis.chan@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Ian Chen <ian.chen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>