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>