drm/nouveau: ensure we've parsed i2c table entry for INIT_*I2C* handlers

We may not have parsed the entry yet if the i2c_index is for an i2c bus
that's not referenced by a DCB encoder.

This could be done oh so much more nicely, except we have to care about
prehistoric DCB tables too, and they make life painful.

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