Allow using vaapi sink when it's possible
We are avoiding of using vaapisink in the streaming+gstvideooverlay case. This issue is possibly fixed already, if not, we may want to avoid of using vaapisink only in the buggy versions of gstreamer (and open a bug in gstreamer if not exist) since the client should preform better when it used.
See create_pipeline in src/channel-display-gst.c