drm/i915: Undo gtt scratch pte unmapping again

drm/i915: Undo gtt scratch pte unmapping again

It apparently blows up on some machines. This functionally reverts

commit 828c79087cec61eaf4c76bb32c222fbe35ac3930
Author: Ben Widawsky <[email protected]>
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 <[email protected]>
Cc: [email protected]
Cc: Takashi Iwai <[email protected]>
Cc: Paulo Zanoni <[email protected]>
Cc: Todd Previte <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>