drm/i915: Drop the extra GEN8_PCU_IIR posting read from cherryview_irq_preinstall()

Looks like a leftover POSTING_READ(GEN8_PCU_IIR) in
cherryview_irq_preinstall() from some earlier age. GEN5_IRQ_RESET()
already does the posting read so this changes nothing, so kill it.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -3188,8 +3188,6 @@ static void cherryview_irq_preinstall(struct drm_device *dev)
 
 	GEN5_IRQ_RESET(GEN8_PCU_);
 
-	POSTING_READ(GEN8_PCU_IIR);
-
 	I915_WRITE(DPINVGTT, DPINVGTT_STATUS_MASK_CHV);
 
 	I915_WRITE(PORT_HOTPLUG_EN, 0);