viafb: enable I2C for CRT

This patch enables the I2C port to talk with the CRT. This allows adding
EDID reading and parsing capability for CRTs.
Just a small preparation, no notable changes in user experience yet.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Cc: Joseph Chan <JosephChan@via.com.tw>