nouveau: restore check that avoids multiple user bos per kernel bo

Lost in 5ea6f1c3, triggering fdo#89842.

Unlike the PRIME fd->handle interfaces, the GEM_OPEN interface doesn't
do anything at the kernel level to prevent this situation occuring,
and we end up with multiple GEM handles for a single kernel buffer.

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