drm/amd/display: 3-plane MPO enablement for DCN321

Enable 3-planes MPO for DCN321 by reporting max_slave_planes in DC caps
for each ASIC.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>