drm/amd/display: Expose 3 secondary planes for supported ASICs

[why]
For enabling 4-plane MPO, we need dc to expose 4 planes for DCN35 and
beyond, as well as DCN21

[how]
Set dc_caps.max_slave_*planes to 3 for appropriate ASICs

Reviewed-by: Sun peng Li <sunpeng.li@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>