drm/i915/dp_mst: Fix race between connector registration and setup

After drm_connector_init() is called the connector is visible to the
rest of the kernel via the drm_mode_config::connector_list. Make
sure that the DSC AUX device and capabilities are setup by that time.

Another race condition is adding the connector to the connector list
before drm_connector_helper_add() sets the connector helper functions.
That's an unrelated issue, for which the fix is for a follow-up. One
solution would be adding the connector to the connector list only
during its registration in drm_connector_register().

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: 808b43fa ("drm/i915/dp_mst: Set connector DSC capabilities and decompression AUX")
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-2-imre.deak@intel.com
(cherry picked from commit 560ea72c)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -1161,6 +1161,14 @@ static struct drm_connector *intel_dp_add_mst_connector(struct drm_dp_mst_topolo
 	intel_connector->port = port;
 	drm_dp_mst_get_port_malloc(port);
 
+	/*
+	 * TODO: set the AUX for the actual MST port decompressing the stream.
+	 * At the moment the driver only supports enabling this globally in the
+	 * first downstream MST branch, via intel_dp's (root port) AUX.
+	 */
+	intel_connector->dp.dsc_decompression_aux = &intel_dp->aux;
+	intel_dp_mst_read_decompression_port_dsc_caps(intel_dp, intel_connector);
+
 	connector = &intel_connector->base;
 	ret = drm_connector_init(dev, connector, &intel_dp_mst_connector_funcs,
 				 DRM_MODE_CONNECTOR_DisplayPort);
@@ -1172,14 +1180,6 @@ static struct drm_connector *intel_dp_add_mst_connector(struct drm_dp_mst_topolo
 
 	drm_connector_helper_add(connector, &intel_dp_mst_connector_helper_funcs);
 
-	/*
-	 * TODO: set the AUX for the actual MST port decompressing the stream.
-	 * At the moment the driver only supports enabling this globally in the
-	 * first downstream MST branch, via intel_dp's (root port) AUX.
-	 */
-	intel_connector->dp.dsc_decompression_aux = &intel_dp->aux;
-	intel_dp_mst_read_decompression_port_dsc_caps(intel_dp, intel_connector);
-
 	for_each_pipe(dev_priv, pipe) {
 		struct drm_encoder *enc =
 			&intel_dp->mst_encoders[pipe]->base.base;