drm/i915: Undo gtt scratch pte unmapping again

It apparently blows up on some machines. This functionally reverts

commit 828c7908
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Wed Oct 16 09:21:30 2013 -0700

    drm/i915: Disable GGTT PTEs on GEN6+ suspend

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=64841Reported-and-Tested-by: Brad  Jackson <bjackson0971@gmail.com>
Cc: stable@vger.kernel.org
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Todd Previte <tprevite@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1339,7 +1339,7 @@ void i915_gem_suspend_gtt_mappings(struct drm_device *dev)
 	dev_priv->gtt.base.clear_range(&dev_priv->gtt.base,
 				       dev_priv->gtt.base.start,
 				       dev_priv->gtt.base.total,
-				       false);
+				       true);
 }
 
 void i915_gem_restore_gtt_mappings(struct drm_device *dev)