drm/i915: Assert that the active request hasn't been signaled

As the request is not complete, it should not be signaled. Assert that
this is true before we process the request for a reset.

References: https://bugs.freedesktop.org/show_bug.cgi?id=99671
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170212172002.23072-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -2611,6 +2611,8 @@ i915_gem_find_active_request(struct intel_engine_cs *engine)
 			continue;
 
 		GEM_BUG_ON(request->engine != engine);
+		GEM_BUG_ON(test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
+				    &request->fence.flags));
 		return request;
 	}