viafb: Only establish i2c busses on ports that always had them

...otherwise it seems we run into conflicts with shadowy other users which
don't expect to see i2c taking control of ports it never used to do
anything with.

Reported-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>