drm/i915: Assert that we do not try to unsubmit a completed request

Assert that we do not try to unsubmit a completed request, as should we
try to resubmit it later, the ring is already past the request's
breadcrumb and the breadcrumb will not be updated.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180129094912.14428-1-chris@chris-wilson.co.ukReviewed-by: Michał Winiarski <michal.winiarski@intel.com>

drivers/gpu/drm/i915/i915_gem_request.c

@@ -537,6 +537,8 @@ void __i915_gem_request_unsubmit(struct drm_i915_gem_request *request)
 	 */
 	GEM_BUG_ON(!request->global_seqno);
 	GEM_BUG_ON(request->global_seqno != engine->timeline->seqno);
+	GEM_BUG_ON(i915_seqno_passed(intel_engine_get_seqno(engine),
+				     request->global_seqno));
 	engine->timeline->seqno--;
 
 	/* We may be recursing from the signal callback of another i915 fence */