drm/i915: Assert that the request is indeed complete when signaled from irq

After we call dma_fence_signal(), confirm that the request was indeed
complete.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180305104105.8296-1-chris@chris-wilson.co.uk