I've sent this patch several times for various reasons. It essentially
cleans up a lot of code where we need to do something per ring, and want
to query whether or not the ring exists on that hardware.

It has various uses coming up, but for now it shouldn't be too
offensive.

v2: Big conflict resolution on Damien's DEV_INFO_FOR_EACH stuff

v3: Resolved vebox addition

v4: Rebased after months of disuse. Also made failed ringbuffer init
cleaner.

v5: Remove the init cleaner from v4. There is a better way to do it.
(Chris)
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>