drm/i915/gt: Move the heartbeat into the high priority system wq

As we ensure that the heartbeat is reasonably fast (and should not
block), move the heartbeat work into the system_highpri_wq to avoid
having this essential task be blocked behind other slow work, such as
our own retire_work_handler.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/2119
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702095219.963-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c

@@ -32,7 +32,7 @@ static bool next_heartbeat(struct intel_engine_cs *engine)
 	delay = msecs_to_jiffies_timeout(delay);
 	if (delay >= HZ)
 		delay = round_jiffies_up_relative(delay);
-	mod_delayed_work(system_wq, &engine->heartbeat.work, delay);
+	mod_delayed_work(system_highpri_wq, &engine->heartbeat.work, delay);
 
 	return true;
 }