viafb: add function to change sync polarity per device

At the moment only the sync polarity for CRT is handled but there are
also bits for controlling the sync polarity for other output devices.
Add a function to change those similar to the other output device
functions.
There is no runtime change yet as the code still handles only CRT.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>