drm/amd/display: Change default policy for MPO with multidisplay

[Why]
Rearranging pipes with multiple displays and multiple planes cannot be
done atomically and requires a much improved sequence to deal with it.

[How]
To workaround such issues, prefer avoid pipe-split policy for
multidisplay scenarios.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>