drm/i915: Use DPINVGTT_STATUS_MASK

Some has given a name for the DPINVGTT status bitmask, so let's use it
instead of the magic number. Looks more like the chv code now.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -3122,7 +3122,7 @@ static void valleyview_irq_preinstall(struct drm_device *dev)
 
 	gen5_gt_irq_reset(dev);
 
-	I915_WRITE(DPINVGTT, 0xff);
+	I915_WRITE(DPINVGTT, DPINVGTT_STATUS_MASK);
 
 	I915_WRITE(PORT_HOTPLUG_EN, 0);
 	I915_WRITE(PORT_HOTPLUG_STAT, I915_READ(PORT_HOTPLUG_STAT));