drm/i915/execlists: Assert the queue is non-empty on unsubmitting

In the sequence

<0>[  531.960431] drv_self-4806    7.... 527402570us : intel_gpu_reset: engine_mask=1, ret=0, retry=0
<0>[  531.960431] drv_self-4806    7.... 527402571us : execlists_reset: rcs0 request global=115de, current=71133
<0>[  531.960431] drv_self-4806    7d..1 527402571us : execlists_cancel_port_requests: rcs0:port0 global=71134 (fence 826b:198), (current 71133)
<0>[  531.960431] drv_self-4806    7d..1 527402572us : execlists_cancel_port_requests: rcs0:port1 global=71135 (fence 826c:53), (current 71133)
<0>[  531.960431] drv_self-4806    7d..1 527402572us : __i915_request_unsubmit: rcs0 fence 826c:53 <- global=71135, current 71133
<0>[  531.960431] drv_self-4806    7d..1 527402579us : __i915_request_unsubmit: rcs0 fence 826b:198 <- global=71134, current 71133
<0>[  531.960431] drv_self-4806    7.... 527402613us : intel_engine_cancel_stop_cs: rcs0
<0>[  531.960431] drv_self-4806    7.... 527402624us : execlists_reset_finish: rcs0

we are missing the execlists_submission_tasklet() invocation before the
execlists_reset_fini() implying that either the queue is empty, or we
failed to schedule and run the tasklet on finish. Add an assert so we
are sure that on unsubmitting the incomplete request after reset, the
queue is indeed populated.
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/20180919195544.1511-12-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -344,6 +344,7 @@ static void __unwind_incomplete_requests(struct intel_engine_cs *engine)
 			last_prio = rq_prio(rq);
 			p = lookup_priolist(engine, last_prio);
 		}
+		GEM_BUG_ON(RB_EMPTY_ROOT(&engine->execlists.queue.rb_root));
 
 		GEM_BUG_ON(p->priority != rq_prio(rq));
 		list_add(&rq->sched.link, &p->requests);