vaapi: fix usage of invalid buffer ids.

Invalid buffer ids are defined by VA_INVALID_ID. Use that through out
vaapi_*.c support files now that we have private data initialized and
managed by libavcodec. Previously, the only requirement for the public
vaapi_context struct was to be zero-initialized.

This fixes support for 3rdparty VA drivers that strictly conform to
the API whereby an invalid buffer id is VA_INVALID_ID and the first
valid buffer id can actually be zero.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
