drm/i915: Clarify event_lock locking, process context

It's good practice to use the more specific versions for irq save
spinlocks both as executable documentation and to enforce saner
design. The _irqsave version really should only be used if the calling
context is unknown and there's a good reason to call a function from
all kinds of places.

This is the first step whice replaces all occurances of _irqsave in
process context with the simpler irq disable/enable variants. We don't
have any funky spinlock nesting going on, especially since the
event_lock is the outermost of the irq/vblank related spinlocks.

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