drm/virtio: Drop dummy save/restore functions

These hooks will be gone soon.

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-11-git-send-email-daniel.vetter@ffwll.ch


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