drm/nouveau/i2c: port to subdev interfaces

v2/v3: Ben Skeggs <bskeggs@redhat.com>
- fix typo in default bus selection
- fix accidental loss of destructor

v4: Dmitry Eremin-Solenikov <dmitry_eremin@mentor.com>
- fix typo causing incorrect default i2c port settings when no BMP data

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