drm/i915: kill ivybridge_irq_preinstall

After Daniel's latest changes it's now equal to
ironlake_irq_preinstall.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>