drm/i915: queue work outside spinlock in hsw_pm_irq_handler

And kill the comment about it. Queueing work is a barrier type event,
no amount of locking will help in ordering things (as long as we queue
the work after having updated all relevant data structures). Also, the
queue_work works itself as a sufficient memory barrier.

Again on the surface this is just a tiny micro-optimization to reduce
the hold-time of dev_priv->irq_lock. But the better reason is that it
reduces superficial locking and so makes it clearer what we actually
need for correctness.
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -969,9 +969,9 @@ static void hsw_pm_irq_handler(struct drm_i915_private *dev_priv,
 		I915_WRITE(GEN6_PMIMR, dev_priv->rps.pm_iir);
 		/* never want to mask useful interrupts. (also posting read) */
 		WARN_ON(I915_READ_NOTRACE(GEN6_PMIMR) & ~GEN6_PM_RPS_EVENTS);
-		/* TODO: if queue_work is slow, move it out of the spinlock */
-		queue_work(dev_priv->wq, &dev_priv->rps.work);
 		spin_unlock(&dev_priv->rps.lock);
+
+		queue_work(dev_priv->wq, &dev_priv->rps.work);
 	}
 
 	if (pm_iir & PM_VEBOX_USER_INTERRUPT)