drm: Move encoder->save/restore into nouveau

Nouveau is the only user, and atomic drivers should do state
save/restoring differently. So move it into noveau.

Saves me typing some kerneldoc, too ;-)

v2: Move misplaced hunk into earlier nouveau patch.

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449245647-1315-1-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Thierry Reding <treding@nvidia.com>