va: decoder: store output buffer rather than surface

GstVaDecodePicture stored the processed VASurfaceID, under the assumption that the bufferpool with keep the referenced buffers, but this approach is fragil.

This patch changes GstVaDecodePicture to store the output buffer, which already contains its VASurfaceID, and provides a new method to retrieve the VASurfaceID directly from picture.

Based on He Junyan junyan.he@intel.com patches for !1587 (merged)