drm/amd/display: Validate mdoe under MST LCT=1 case as well

[Why & How]
Currently in dm_dp_mst_is_port_support_mode(), when valdidating mode
under dsc decoding at the last DP link config, we only validate the
case when there is an UFP. However, if the MSTB LCT=1, there is no
UFP.

Under this case, use root_link_bw_in_kbps as the available bw to
compare.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/3720
Fixes: fa57924 ("drm/amd/display: Refactor function dm_dp_mst_is_port_support_mode()")
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1835,11 +1835,15 @@ enum dc_status dm_dp_mst_is_port_support_mode(
 			if (immediate_upstream_port) {
 				virtual_channel_bw_in_kbps = kbps_from_pbn(immediate_upstream_port->full_pbn);
 				virtual_channel_bw_in_kbps = min(root_link_bw_in_kbps, virtual_channel_bw_in_kbps);
-				if (bw_range.min_kbps > virtual_channel_bw_in_kbps) {
-					DRM_DEBUG_DRIVER("MST_DSC dsc decode at last link."
-							 "Max dsc compression can't fit into MST available bw\n");
-					return DC_FAIL_BANDWIDTH_VALIDATE;
-				}
+			} else {
+				/* For topology LCT 1 case - only one mstb*/
+				virtual_channel_bw_in_kbps = root_link_bw_in_kbps;
+			}
+
+			if (bw_range.min_kbps > virtual_channel_bw_in_kbps) {
+				DRM_DEBUG_DRIVER("MST_DSC dsc decode at last link."
+						 "Max dsc compression can't fit into MST available bw\n");
+				return DC_FAIL_BANDWIDTH_VALIDATE;
 			}
 		}