accel: update vaapi

Most of the logic for vaapi has been moved to lavc, so update Ring
to reflect these changes. This also removes a compilation warning
related to a deprecated struct.

Change-Id: I16e444c49fb358184f20c7211365c6ac95fc03a1
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>