drm/amd/display: Add minimal pipe split transition state

[WHY?] When adding/removng a plane to some configurations,
unsupported pipe programming can occur when moving to a new plane.

[HOW?]
Add a safe transistion state before programming new configuration.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>