vaapivideomemory: release proxy's data if downstream
vaapivideomemory: release proxy's data if downstream

The surface created for downstream is going to be filled by VAAPI
elements. So, the driver needs write access on that surface.

This patch releases the derived image held by the proxy, thus the
surface is unmarked as busy.

This is how it has to be done as discussed on libva mailing list.

https://bugzilla.gnome.org/show_bug.cgi?id=755072

GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi