drm/i915: Skip request serialisation if the timeline is already complete

If the last request on the timeline is already complete, we do not need
to emit the serialisation barriers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207084350.3929-2-chris@chris-wilson.co.uk