drm/nouveau/nvkm: support loading fws into sg_table

- preparation for GSP-RM, which has massive FW images
- based on a patch by Dave Airlie

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230918202149.4343-32-skeggsb@gmail.com