drm/amd/display: Fix SubVP control flow in the MPO context

SubVP has some issues related to how we allocate and enable it. This
commit fixes this behavior by adding the proper check and configuration
to the SubVP code path.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>