drm/dp_mst: Fix flushing the delayed port/mstb destroy work

Atm, a pending delayed destroy work during module removal will be
canceled, leaving behind MST ports, mstbs. Fix this by using a dedicated
workqueue which will be drained of requeued items as well when
destroying it.

v2:
- Check if wq is NULL before calling destroy_workqueue().

Cc: Lyude Paul <lyude@redhat.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200610134704.25270-1-imre.deak@intel.com

include/drm/drm_dp_mst_helper.h

@@ -681,6 +681,14 @@ struct drm_dp_mst_topology_mgr {
 	 * @destroy_branch_device_list.
 	 */
 	struct mutex delayed_destroy_lock;
+
+	/**
+	 * @delayed_destroy_wq: Workqueue used for delayed_destroy_work items.
+	 * A dedicated WQ makes it possible to drain any requeued work items
+	 * on it.
+	 */
+	struct workqueue_struct *delayed_destroy_wq;
+
 	/**
 	 * @delayed_destroy_work: Work item to destroy MST port and branch
 	 * devices, needed to avoid locking inversion.