drm/i915: fix typo in display IRQ mask when disabling IRQs

Introduced in
commit e0e33f8ff6f0b6d286afc314802be4993341bd47
Author: Imre Deak <imre.deak@intel.com>
Date:   Tue Mar 4 19:23:07 2014 +0200

The impact was luckily minimal, due to the extra check we do against a
software pipestat IRQ mask.

Caught by Fengguang's 0-day tester.

Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -3074,7 +3074,7 @@ static void valleyview_display_irqs_uninstall(struct drm_i915_private *dev_priv)
 
 	iir_mask = I915_DISPLAY_PORT_INTERRUPT |
 		   I915_DISPLAY_PIPE_A_EVENT_INTERRUPT |
-		   I915_DISPLAY_PIPE_A_EVENT_INTERRUPT;
+		   I915_DISPLAY_PIPE_B_EVENT_INTERRUPT;
 
 	dev_priv->irq_mask |= iir_mask;
 	I915_WRITE(VLV_IER, ~dev_priv->irq_mask);