drm/nouveau/core/object: pass client directly to ioctl handlers

nvkm_object::client refers to the client that created the object, which,
is currently always the same as the ioctl caller.

Upcoming patches introduce the concept of subclients, where a parent is
able to access the object trees of its children, making the above no
longer true.

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