drm/i915: Consolidate the timeline->barrier

The timeline is strictly ordered, so by inserting the timeline->barrier
request into the timeline->last_request it naturally provides the same
barrier. Consolidate the pair of barriers into one as they serve the
same purpose.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190408091728.20207-4-chris@chris-wilson.co.uk