drm/i915: move hpd handling to (ibx|cpt)_irq_handler

Somehow this was left out in the refactoring that introduced the pch
handlers. Avoids a hotplug_mask special case in the ilk_irq_handler.

Noticed while hunting down the pch hotplug bits.

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