drm/i915: add ibx_irq_postinstall

So we can remove duplicated code. Note that this function is used not
only on IBX, but also CPT and LPT.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Also bikeshed s/ironlake_enable_pch_hotplug/ibx_enable_hotplug
to keep consistent with our ibx for pch naming scheme.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>