vnc: return directly if no vnc client connected

graphic_hw_update and vnc_refresh_server_surface aren't
need to do when no vnc client connected. It can reduce
lock contention, because vnc_refresh will hold global big
lock two millisecond every three seconds.
Signed-off-by: NChenLiang <chenliang88@huawei.com>
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

@@ -2778,6 +2778,11 @@ static void vnc_refresh(DisplayChangeListener *dcl)
     VncState *vs, *vn;
     int has_dirty, rects = 0;
 
+    if (QTAILQ_EMPTY(&vd->clients)) {
+        update_displaychangelistener(&vd->dcl, VNC_REFRESH_INTERVAL_MAX);
+        return;
+    }
+
     graphic_hw_update(NULL);
 
     if (vnc_trylock_display(vd)) {
@@ -2793,11 +2798,6 @@ static void vnc_refresh(DisplayChangeListener *dcl)
         /* vs might be free()ed here */
     }
 
-    if (QTAILQ_EMPTY(&vd->clients)) {
-        update_displaychangelistener(&vd->dcl, VNC_REFRESH_INTERVAL_MAX);
-        return;
-    }
-
     if (has_dirty && rects) {
         vd->dcl.update_interval /= 2;
         if (vd->dcl.update_interval < VNC_REFRESH_INTERVAL_BASE) {