drm/i915: Check signaled state after enabling signaling

Setting up the irq to signal the request completion takes a finite
amount of time, during which it is possible that the request already
completed. Check afterwards, just in case, so that we can respond
immediately.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170608111405.16466-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_request.c

@@ -62,7 +62,7 @@ static bool i915_fence_enable_signaling(struct dma_fence *fence)
 		return false;
 
 	intel_engine_enable_signaling(to_request(fence), true);
-	return true;
+	return !i915_fence_signaled(fence);
 }
 
 static signed long i915_fence_wait(struct dma_fence *fence,