drm/dp_mst: Sanitize mgr->qlock locking in drm_dp_mst_wait_tx_reply()

Make the locking look symmetric with the unlocking.
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200603211040.8190-2-imre.deak@intel.com

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -1183,7 +1183,7 @@ static int drm_dp_mst_wait_tx_reply(struct drm_dp_mst_branch *mstb,
 	ret = wait_event_timeout(mgr->tx_waitq,
 				 check_txmsg_state(mgr, txmsg),
 				 (4 * HZ));
-	mutex_lock(&mstb->mgr->qlock);
+	mutex_lock(&mgr->qlock);
 	if (ret > 0) {
 		if (txmsg->state == DRM_DP_SIDEBAND_TX_TIMEOUT) {
 			ret = -EIO;