drm/amd/display: Add code comments clock and encode code

drm/amd/display: Add code comments clock and encode code

This commit adds some comments to make easier to understand the clock
update for DCN 201, the encode function, and other minor comments.

Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>