drm/i915/gt: Always reschedule the new heartbeat

In order to better respond to new heartbeat intervals given via sysfs,
always reprogramme an active heartbeat upon change (i.e. use
mod_delayed_work to reschedule rather than queue_delayed_work which
ignores an already active work.)

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/20200317163208.30010-1-chris@chris-wilson.co.uk

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

@@ -31,7 +31,7 @@ static bool next_heartbeat(struct intel_engine_cs *engine)
 	delay = msecs_to_jiffies_timeout(delay);
 	if (delay >= HZ)
 		delay = round_jiffies_up_relative(delay);
-	schedule_delayed_work(&engine->heartbeat.work, delay);
+	mod_delayed_work(system_wq, &engine->heartbeat.work, delay);
 
 	return true;
 }