drm/nouveau/i2c: balance port acquire/release

This was a half-finished hack before, just enough to handle the shared
aux/i2c pad thing on G94 and up.

We got lucky with locking etc up until now, as this was (generally) all
protected by the DRM mode_config lock.  It's about to become a lot more
likely to hit the races.

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