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>

drivers/gpu/drm/amd/display/dc/dcn31/dcn31_resource.c

@@ -1868,9 +1868,9 @@ static bool dcn31_resource_construct(
 	dc->caps.min_horizontal_blanking_period = 80;
 	dc->caps.dmdata_alloc_size = 2048;
 
-	dc->caps.max_slave_planes = 1;
-	dc->caps.max_slave_yuv_planes = 1;
-	dc->caps.max_slave_rgb_planes = 1;
+	dc->caps.max_slave_planes = 2;
+	dc->caps.max_slave_yuv_planes = 2;
+	dc->caps.max_slave_rgb_planes = 2;
 	dc->caps.post_blend_color_processing = true;
 	dc->caps.force_dp_tps4_for_cp2520 = true;
 	dc->caps.dp_hpo = true;