drm/i915: Silence an innocuous compiler warning for an unused variable

drivers/gpu/drm/i915/i915_irq.c: In function ‘ironlake_irq_postinstall’:
drivers/gpu/drm/i915/i915_irq.c:1618: warning: unused variable ‘pipe’

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>