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=64841


Reported-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>
Signed-off-by: Dave Airlie <airlied@redhat.com>