drm/amd/display: Enable 3-plane MPO for DCN31

[WHY&HOW]
It can be enabled by users, but proper way is to report max_slave_planes
in DC caps for each ASIC.
Some structures use hardcoded max_plane=2, this is also addressed here.

Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Alex Hung <alex.hung@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>