Use GstVaapiUploader helper that automatically handles direct rendering
mode, thus making the "direct-rendering" property obsolete and hence it
is now removed.
The "direct-rendering" level 2, i.e. exposing VA surface buffers, was never
really well supported and it could actually trigger degraded performance.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>