drm/amd/display: add getter routine to retrieve mpcc mux

[Why & How]
Add function to identify which MPCC is providing input to a specified OPP

Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>