drm/amd/display: Change DET policy for MPO cases

For MPO we want to allocate less than maximum DET for MPO pipes because
we need enogh buffer to move DET back to other pipes when removing an
MPO plane. Also update regular DET allocation to use DET override (DCN32
has an internal policy which driver does not want to use)

Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>