drm/i915: don't update GEN6_PMIMR when it's not needed

I did some brief tests and the "new_val = pmimr" condition usually
happens a few times after exiting games.

Note: This is also prep work to track the GEN6_PMIMR register state in
dev_priv->pm_imr. This happens in the next patch.
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Add note to explain why we want this, as per the discussion
between Chris and Paulo.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -142,14 +142,18 @@ static void snb_update_pm_irq(struct drm_i915_private *dev_priv,
 			      uint32_t interrupt_mask,
 			      uint32_t enabled_irq_mask)
 {
-	uint32_t pmimr = I915_READ(GEN6_PMIMR);
-	pmimr &= ~interrupt_mask;
-	pmimr |= (~enabled_irq_mask & interrupt_mask);
+	uint32_t pmimr, new_val;
 
 	assert_spin_locked(&dev_priv->irq_lock);
 
-	I915_WRITE(GEN6_PMIMR, pmimr);
-	POSTING_READ(GEN6_PMIMR);
+	pmimr = new_val = I915_READ(GEN6_PMIMR);
+	new_val &= ~interrupt_mask;
+	new_val |= (~enabled_irq_mask & interrupt_mask);
+
+	if (new_val != pmimr) {
+		I915_WRITE(GEN6_PMIMR, new_val);
+		POSTING_READ(GEN6_PMIMR);
+	}
 }
 
 void snb_enable_pm_irq(struct drm_i915_private *dev_priv, uint32_t mask)