drm/i915: POSTING_READ(DEIER) on ivybridge_irq_handler

We have this POSTING_READ inside ironlake_irq_handler. I suppose we
also want it on IVB because we want to stop the IRQ handler as soon as
possible at this point.

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