drm/amd/display: Update some comments to improve the code readability

This commit updates some comments to be more precise and adds another
small comment to some other parts to improve the code readability.

Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>