drm/dp/mst: take lock around looking up the branch device on hpd irq

If we are doing an MST transaction and we've gotten HPD and we
lookup the device from the incoming msg, we should take the mgr
lock around it, so that mst_primary and mstb->ports are valid.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -1163,6 +1163,8 @@ static struct drm_dp_mst_branch *drm_dp_get_mst_branch_device(struct drm_dp_mst_
 	struct drm_dp_mst_port *port;
 	int i;
 	/* find the port by iterating down */
+
+	mutex_lock(&mgr->lock);
 	mstb = mgr->mst_primary;
 
 	for (i = 0; i < lct - 1; i++) {
@@ -1182,6 +1184,7 @@ static struct drm_dp_mst_branch *drm_dp_get_mst_branch_device(struct drm_dp_mst_
 		}
 	}
 	kref_get(&mstb->kref);
+	mutex_unlock(&mgr->lock);
 	return mstb;
 }