drm/i915: Execute signal callbacks from no-op i915_request_wait

If we enter i915_request_wait() with an already completed request, but
unsignaled dma-fence, signal the fence before returning. This allows us
to execute any of the signal callbacks at the earliest opportunity.

v2: Also signal after busyspin success

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/20190614111053.25615-2-chris@chris-wilson.co.uk