drm/amd/display: Fix construct_phy with MXM connector

[Why/How]
The call to construct_phy will fail in cases where connector type is
MXM, and the dc_link won't be properly created/initialized.
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Signed-off-by: Roman Li <roman.li@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/link/link_factory.c

@@ -524,6 +524,7 @@ static bool construct_phy(struct dc_link *link,
 		link->connector_signal = SIGNAL_TYPE_DVI_DUAL_LINK;
 		break;
 	case CONNECTOR_ID_DISPLAY_PORT:
+	case CONNECTOR_ID_MXM:
 	case CONNECTOR_ID_USBC:
 		link->connector_signal = SIGNAL_TYPE_DISPLAY_PORT;