drm/i915: add dev_priv->pm_irq_mask

Just like irq_mask and gt_irq_mask, use it to track the status of
GEN6_PMIMR so we don't need to read it again every time we call
snb_update_pm_irq.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 9bf2801..f423a94 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1123,6 +1123,7 @@
 	/** Cached value of IMR to avoid reads in updating the bitfield */
 	u32 irq_mask;
 	u32 gt_irq_mask;
+	u32 pm_irq_mask;
 
 	struct work_struct hotplug_work;
 	bool enable_hotplug_processing;