drm/amd/display: Added documentation for some DC interface functions

Signed-off-by: Yasir Al Shekerchi <YasirAl.Shekerchi@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>