drm/amd/display: Add PBN per slot calculation for DSC

[why]
Need to calculate VCPI slots differently for DSC
to take in account current link rate, link count
and FEC.
[how]
Add helper to get pbn_div from dc_link
Acked-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c

@@ -493,3 +493,11 @@ void amdgpu_dm_initialize_dp_connector(struct amdgpu_display_manager *dm,
 		aconnector->connector_id);
 }
 
+int dm_mst_get_pbn_divider(struct dc_link *link)
+{
+	if (!link)
+		return 0;
+
+	return dc_link_bandwidth_kbps(link,
+			dc_link_get_link_cap(link)) / (8 * 1000 * 54);
+}

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.h

@@ -29,6 +29,8 @@
 struct amdgpu_display_manager;
 struct amdgpu_dm_connector;
 
+int dm_mst_get_pbn_divider(struct dc_link *link);
+
 void amdgpu_dm_initialize_dp_connector(struct amdgpu_display_manager *dm,
 				       struct amdgpu_dm_connector *aconnector);