drm/nouveau/fb/gf100-: allow selection of an alternate big page size

GFxxx/GM1xx support the selection of 64/128KiB big pages globally.

GM2xx supports the same, as well as another mode where the page size
can be selected per-instance.

We default to 128KiB pages (With per-instance for GM200, but the current
code selects 128KiB there already) as the MMU code isn't currently able
to handle otherwise.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>