This was once used by userspace tools (with nvkm built as a library), as
a way to select a "default device".

The DRM code doesn't need this at all as clients only have access to a
single device already, so inherit the value from its parent.
Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240726043828.58966-20-bskeggs@nvidia.com