VA: Add the aux surface for gst buffer used by decoder.

VA: Add the aux surface for gst buffer used by decoder.

The AV1 codec needs to support the film grain feature. When the film
grain feature is enabled, we need two surfaces as the output of the
decoded picture, one without film grain effect and the other one with
it. The first one acts as the reference and is needed for later pictures'
reconstruction, and the second one is the real display output.
So we need to attach another aux surface to the gst buffer/mem and make
that aux surface as the target of vaBeginPicture.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1636>