drm/nouveau: omit to create schedulers using the legacy uAPI

Omit to create scheduler instances when using the legacy uAPI. When
using the legacy NOUVEAU_GEM_PUSHBUF ioctl no scheduler instance is
required, hence omit creating scheduler instances in
nouveau_abi16_ioctl_channel_alloc().

Tested-by: Timur Tabi <ttabi@nvidia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240202000606.3526-2-dakr@redhat.com