plugins: don't reallocate pool allocator for the same caps.
plugins: don't reallocate pool allocator for the same caps.

If the video buffer pool config doesn't have new caps, then it's not
necessary to reinstantiate the allocator. That could be a costly
operation as we could do some extra heavy checking in there.

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