drm/amd/display: Allow 16 max_slices for DP2 DSC

Enable 12 and 16 max_slices for DP2 DSC

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>