drm/amd/display: properly turn off unused mpc before front end programming

MPCC_OPP_ID must be programmed to 0xf to properly turn off the mpcc.
However the software state of the mpcc must keep track of the opp that
the mpcc is attached to for reset to properly happen. This is kinda
hacky right now, but a good solution may involve a lot of work.

Signed-off-by: Eric Yang <Eric.Yang2@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>