vaapivideobufferpool: set correct buffer size

vaapivideobufferpool: set correct buffer size

We should set the correct buffer size when we are configuring the pool,
otherwise the buffer will be discarded when it returns to the pool.

Indeed when the ref-count of a buffer reaches zero, its pool will queue
it back (and ref it) if, and only if, the buffer size matches the
configured buffer size on the pool.

This issue can be debugged with GST_DEBUG=*PERF*:6, see gstbufferpool.c

https://bugzilla.gnome.org/show_bug.cgi?id=774782