drm/amd/display: Enable MPO with pre-blend color processing (RGB)

drm/amd/display: Enable MPO with pre-blend color processing (RGB)

[Why]
DCN10 performs color processing before MPC combination, causes color
shift in RGB colorspaces when positive brightness offset is applied
However, YCbCr is still unfixed and remains disabled

[How]
Add layerIndex to dc_plane_state and dc_plane_info structs
Re-enable MPO when brightness is adjusted and colorspace is not YCbCr
Set rear plane's brightness offset to 0 when front plane visible

Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>