drm/i915/bdw: make sure south port interrupts are enabled properly v2

We were apparently relying on the defaults on BDW, which resulted in no
hotplug or AUX interrupts.  So be sure to call the ibx_irq_preinstall to
enable all interrupts.

v2: use preinstall instead of redundant SDIER write

References: https://bugs.freedesktop.org/show_bug.cgi?id=72834
References: https://bugs.freedesktop.org/show_bug.cgi?id=72833
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>