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>

drivers/gpu/drm/i915/i915_irq.c

@@ -2713,6 +2713,8 @@ static void gen8_irq_preinstall(struct drm_device *dev)
 #undef GEN8_IRQ_INIT_NDX
 
 	POSTING_READ(GEN8_PCU_IIR);
+
+	ibx_irq_preinstall(dev);
 }
 
 static void ibx_hpd_irq_setup(struct drm_device *dev)