drm/nouveau/ttm: handle ttm moves properly.

The idea is to flip the core over to calling the driver always,
so add support for moves here.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200924051845.397177-6-airlied@gmail.com