drm/gma500: Activate the gtt rebuild on suspend/resume

This patch activates the rebuilding of the gtt. Currently we reinitialize the
gtt by inserting the stolen pages again and map the rest to our scratch page.
Then we go about restoring the needed ranges. This is a bit overkill but right
now we don't have that much to restore so better safe than sorry.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>