drm/i915: Assert all timeline requests are gone before fini

During i915_gem_timeline_fini(), assert that all the timeline's request
are completed and removed from the timeline.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170105153023.30575-1-chris@chris-wilson.co.uk