As we check that the ringbuffer will not wrap upon emission, we do not
need to check that incrementing the tail wrapped every time. However, we
do upon advancing just in case the tail is now pointing at the very end
of the ring.

Likewise we can account for the space used during emission in begin()
and avoid decrementing it for every emit.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>