drm/dp_mst: Add drm_dp_mst_aux_for_parent()

Add a function to get the AUX device of the parent of an MST port, used
by a follow-up i915 patch in the patchset.

v2: Move drm_dp_mst_aux_for_parent() forward declaration to this patch
    (Ankit)

Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416221010.376865-10-imre.deak@intel.com

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -6010,6 +6010,22 @@ static bool drm_dp_mst_is_virtual_dpcd(struct drm_dp_mst_port *port)
 	return false;
 }
 
+/**
+ * drm_dp_mst_aux_for_parent() - Get the AUX device for an MST port's parent
+ * @port: MST port whose parent's AUX device is returned
+ *
+ * Return the AUX device for @port's parent or NULL if port's parent is the
+ * root port.
+ */
+struct drm_dp_aux *drm_dp_mst_aux_for_parent(struct drm_dp_mst_port *port)
+{
+	if (!port->parent || !port->parent->port_parent)
+		return NULL;
+
+	return &port->parent->port_parent->aux;
+}
+EXPORT_SYMBOL(drm_dp_mst_aux_for_parent);
+
 /**
  * drm_dp_mst_dsc_aux_for_port() - Find the correct aux for DSC
  * @port: The port to check. A leaf of the MST tree with an attached display.

include/drm/display/drm_dp_mst_helper.h

@@ -955,6 +955,7 @@ bool drm_dp_mst_port_is_logical(struct drm_dp_mst_port *port)
 	return port->port_num >= DP_MST_LOGICAL_PORT_0;
 }
 
+struct drm_dp_aux *drm_dp_mst_aux_for_parent(struct drm_dp_mst_port *port);
 struct drm_dp_aux *drm_dp_mst_dsc_aux_for_port(struct drm_dp_mst_port *port);
 
 static inline struct drm_dp_mst_topology_state *