drm/i915: consolidate ring->sync-to functions

The waiter is always the ring itself (otherwise we'd have a decent
snafu in a callsite), so we can unify this easily.

Also give it the usual gen6_ prefix, in case anyone is foolish enough to
implement hw semaphores for gen5.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>