drm/i915: Clarify irq_lock locking, irq handlers

irq handlers always run with interrupts locally disabled, so
plain spinlocks is all we need. I've also reviewed again that they
all follow the _irq_handler postfix convention.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>