drm/i915: add ibx_irq_preinstall

So we can remove some duplicate code. All the PCHs are very similar
and right now the code is the same. I plan to add more code, so we
would have more duplicated code.

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