plugins: simplify VA video meta to only reference surface proxies.
plugins: simplify VA video meta to only reference surface proxies.

Simplify GstVaapiVideoMeta to only hold a surface proxy, which is
now allocated from a surface pool. This also means that the local
reference to the VA surface is also gone, as it could be extracted
from the associated surface proxy.

01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi