drm/i915/selftests: Flush any i915_active callback work as well

Make trebly sure that all possible callbacks and their delayed brethren
are complete before asserting that the i915_active should be idle after
flushing all barriers.
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/20191023235359.27132-1-chris@chris-wilson.co.uk

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

@@ -55,6 +55,7 @@ static void pulse_unlock_wait(struct pulse *p)
 {
 	mutex_lock(&p->active.mutex);
 	mutex_unlock(&p->active.mutex);
+	flush_work(&p->active.work);
 }
 
 static int __live_idle_pulse(struct intel_engine_cs *engine,