drm/i915/execlists: Mark up priority boost on preemption

Record the priority boost we giving to the preempted client or else we
may end up in a situation where the priority queue no longer matches the
request priority order and so we can end up in an infinite loop of
preempting the same pair of requests.

Fixes: e9eaf82d ("drm/i915: Priority boost for waiting clients")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190123135155.21562-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -302,6 +302,7 @@ static void __unwind_incomplete_requests(struct intel_engine_cs *engine)
 	 */
 	if (!(prio & I915_PRIORITY_NEWCLIENT)) {
 		prio |= I915_PRIORITY_NEWCLIENT;
+		active->sched.attr.priority = prio;
 		list_move_tail(&active->sched.link,
 			       i915_sched_lookup_priolist(engine, prio));
 	}
@@ -625,6 +626,9 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		int i;
 
 		priolist_for_each_request_consume(rq, rn, p, i) {
+			GEM_BUG_ON(last &&
+				   need_preempt(engine, last, rq_prio(rq)));
+
 			/*
 			 * Can we combine this request with the current port?
 			 * It has to be the same context/ringbuffer and not