drm/i915: Rename engine->semaphore.sync_to, engine->sempahore.signal locals

In order to be more consistent with the rest of the request construction
and ring emission, use the common names for the ring and request.

Rather than using signaler_req, waiter_req, and intel_ring *wait, we use
plain req and ring.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-32-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-23-git-send-email-chris@chris-wilson.co.uk