drm/gma500: fix things after get/put page helpers

Commit 8b9ba7a3 'drm/gma500: use gem get/put page helpers' was missing a
line, which resulted in garbled screen.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Tested-by: Guillaume CLÉMENT <gclement@baobob.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>