drm/i915: Save/restore HWS_PGA on suspend/resume

It fixes suspend/resume failure of xf86-video-intel dri2
branch. As dri2 branch doesn't call I830DRIResume() to restore
hardware status page anymore, we need to preserve
this register across suspend/resume.
Signed-off-by: Peng Li <peng.li@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -150,6 +150,7 @@ typedef struct drm_i915_private {
 	u32 saveDSPBCNTR;
 	u32 saveDSPARB;
 	u32 saveRENDERSTANDBY;
+	u32 saveHWS;
 	u32 savePIPEACONF;
 	u32 savePIPEBCONF;
 	u32 savePIPEASRC;

drivers/gpu/drm/i915/i915_suspend.c

@@ -244,6 +244,9 @@ int i915_save_state(struct drm_device *dev)
 	if (IS_I965G(dev) && IS_MOBILE(dev))
 		dev_priv->saveRENDERSTANDBY = I915_READ(MCHBAR_RENDER_STANDBY);
 
+	/* Hardware status page */
+	dev_priv->saveHWS = I915_READ(HWS_PGA);
+
 	/* Display arbitration control */
 	dev_priv->saveDSPARB = I915_READ(DSPARB);
 
@@ -373,6 +376,9 @@ int i915_restore_state(struct drm_device *dev)
 	if (IS_I965G(dev) && IS_MOBILE(dev))
 		I915_WRITE(MCHBAR_RENDER_STANDBY, dev_priv->saveRENDERSTANDBY);
 
+	/* Hardware status page */
+	I915_WRITE(HWS_PGA, dev_priv->saveHWS);
+
 	/* Display arbitration */
 	I915_WRITE(DSPARB, dev_priv->saveDSPARB);