drm/amd/display: Add comments to improve the code readability

This commit just introduce some basic comments that helps to understand
the overall behavior of some structs.

Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>