drm/i915: Clarify event_lock locking, irq&mixed context

Now we tackle the functions also called from interrupt handlers.

- intel_check_page_flip is exclusively called from irq handlers, so a
  plain spin_lock is all we need. In i915_irq.c we have the convention
  to give all such functions an _irq_handler postfix, but that would
  look strange and als be a bit a misleading name. I've opted for a
  WARN_ON(!in_irq()) instead.

- The other two places left are called both from interrupt handlers
  and from our reset work, so need the full irqsave dance. Annotate
  them with a short comment.

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