drm/i915: irq handlers don't need interrupt-safe spinlocks

Since we only have one interrupt handler and interrupt handlers are
non-reentrant.

To drive the point really home give them all an _irq_handler suffix.

This is a tiny micro-optimization but even more important it makes it
clearer what locking we actually need. And in case someone screws this
up: lockdep will catch hardirq vs. other context deadlocks.

v2: Fix up compile fail.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>