drm/gma500: Move to private save/restore hooks

I want to remove the core ones since with atomic drivers system
suspend/resume is solved much differently. And there's only 2 drivers
(nouveau besides gma500) really using them.

v2: Fixup build noise 0day reported.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-13-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: Thierry Reding <treding@nvidia.com> (v1)