drm/nouveau: handle limited nvif ioctl in abi16

drm/nouveau: handle limited nvif ioctl in abi16

nouveau_usif.c was already stripped right back a couple of years ago,
limiting what userspace could do with it.

A follow-on series removes the nvkm side of these interfaces entirely,
in order to make it less of a nightmare to add/change internal APIs in
the future.

Unfortunately.  Userspace uses some of this.

Fortunately, userspace only ever ended up using a fraction of the APIs,
so those are reimplemened here in a more direct manner, and return
-EINVAL to userspace for everything else.

v2:
- simplified struct nouveau_abi16_obj
- added a couple of comments
v3:
- comment harder

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240726043828.58966-9-bskeggs@nvidia.com