drm/i915: Inline I915_INTERRUPT_ENABLE_FIX

Since there is only one remaining user of I915_INTERRUPT_ENABLE_FIX,
expand it at the callsite. Quoting Jesse Barnes:

"I'd really like to get rid of these defines at the top of i915_irq.c.
Some are unused and the others just make you check for the right bits
everytime your read the code."

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Add bikeshed suggested by Jesse.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>