drm/i915: Drop useless VLV_IIR writes from vlv_display_irq_postinstall()

The extra VLV_IIR writes at the end of vlv_display_irq_postinstall()
serve no purpose. Remove them.

The VLV_IMR/IER/IIR setup at the start of the function also seems a bit
pointless since it doesn't unmask/enable anything. But leave it be for
now.
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

@@ -3444,9 +3444,6 @@ static void vlv_display_irq_postinstall(struct drm_i915_private *dev_priv)
 	if (dev_priv->display_irqs_enabled)
 		valleyview_display_irqs_install(dev_priv);
 	spin_unlock_irq(&dev_priv->irq_lock);
-
-	I915_WRITE(VLV_IIR, 0xffffffff);
-	I915_WRITE(VLV_IIR, 0xffffffff);
 }
 
 static int valleyview_irq_postinstall(struct drm_device *dev)