nouveau: make use of nouveau_drm::fd instead of nouveau_device::fd

The latter is deprecated, and will not be valid for newer clients.

v2.
- split out nouveau_object_find removal

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>