drm/dp_mst: Fix DSC decompression detection in Synaptics branch devices

Atm it's assumed that all Synaptics MST branch devices support DSC,
which is not exposed via a DP-to-DP peer device, rather a control flag
in the branch device's UFP DPCD applying to all the streams going
through it. This isn't true for all adapters with this branch device
though (for instance the Cakitte USBC->2xHDMI adapter reported in the
Closes link below doesn't support DSC).

Fix the above by advertising the DSC decompression support only if the
capability flag for this in the UFP DPCD is actually set.

Cc: Lyude Paul <[e-mail]>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12047
Signed-off-by: Imre Deak <[e-mail]>
Reviewed-by: Lyude Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/display/drm_dp_mst_topology.c

···
 	struct drm_dp_aux *immediate_upstream_aux;
 	struct drm_dp_mst_port *fec_port;
 	struct drm_dp_desc desc = {};
+	u8 upstream_dsc;
 	u8 endpoint_fec;
 	u8 endpoint_dsc;
 
···
 
 	/* DP-to-DP peer device */
 	if (drm_dp_mst_is_virtual_dpcd(immediate_upstream_port)) {
-		u8 upstream_dsc;
-
 		if (drm_dp_dpcd_read(&port->aux,
 				     DP_DSC_SUPPORT, &endpoint_dsc, 1) != 1)
 			return NULL;
···
 
 	if (drm_dp_has_quirk(&desc, DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD)) {
 		u8 dpcd_ext[DP_RECEIVER_CAP_SIZE];
+
+		if (drm_dp_dpcd_read(immediate_upstream_aux,
+				     DP_DSC_SUPPORT, &upstream_dsc, 1) != 1)
+			return NULL;
+
+		if (!(upstream_dsc & DP_DSC_DECOMPRESSION_IS_SUPPORTED))
+			return NULL;
 
 		if (drm_dp_read_dpcd_caps(immediate_upstream_aux, dpcd_ext) < 0)
 			return NULL;