nouveau: introduce object to represent the kernel client

Because NVIF intentionally lacks some of the paths necessary to be
compatible with various mistakes we've made over the years, libdrm
needs to know whether a client has been updated and that it's safe
to make use of the new kernel interfaces.

Clients still using nouveau_device_open()/wrap() will be forced to
make use of ABI16 instead of NVIF.

v2.
- remove lib_version, nothing used it
- leave client-provided pointer unmodified on failure

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>