drm/i915: Ensure cache flushes prior to doing CS flips

Synchronising to an object active on the same ring is a no-op, for the
benefit of execbuffer scheduler. However, for CS flips this means that
we can forgo checking whether the last write request of the object is
actually queued and more importantly whether the cache flush for the
write was emitted.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>