drm/i915: Kill leftover GTIIR writes from valleyview_irq_preinstall()

There are two leftover GTIIR writes in valleyview_irq_preinstall().
Looks like the were originally left behind by:

 commit d18ea1b
 Author: Daniel Vetter <daniel.vetter@ffwll.ch>
 Date:   Fri Jul 12 22:43:25 2013 +0200

    drm/i915: unify PM interrupt preinstall sequence

and then the GTIIR reset was added back here:

 commit f86f3fb
 Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
 Date:   Tue Apr 1 15:37:14 2014 -0300

    drm/i915: properly clear IIR at irq_uninstall on Gen5+

so we can kill the leftovers from the vlv code.

Cc: Paulo Zanoni <przanoni@gmail.com>
Suggested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -3128,10 +3128,6 @@ static void valleyview_irq_preinstall(struct drm_device *dev)
 	I915_WRITE(RING_IMR(GEN6_BSD_RING_BASE), 0);
 	I915_WRITE(RING_IMR(BLT_RING_BASE), 0);
 
-	/* and GT */
-	I915_WRITE(GTIIR, I915_READ(GTIIR));
-	I915_WRITE(GTIIR, I915_READ(GTIIR));
-
 	gen5_gt_irq_reset(dev);
 
 	I915_WRITE(DPINVGTT, DPINVGTT_STATUS_MASK);