drm/i915: Signal fence completion from i915_request_wait

With the upcoming change to automanaged i915_active, the intent is that
whenever we wait on the set of active fences, they are signaled and
collected.  The requirement is that all successful returns from
i915_request_wait() signal the fence, so fixup the one remaining path
where we may return before the interrupt has been run.

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/20190619112341.9082-3-chris@chris-wilson.co.uk