drm/nouveau/gr/gf100-: make global pagepool actually global

This was thought to be per-channel initially - it's not.  The backing
pages for the VMM mappings are shared for all channels.

- switches to more straight-forward patch interfaces
- prepares for sub-context support

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>