viafb: set sync polarity for all output devices

This patch sets the sync polarity for all output devices, not only CRT.
This may give some people a working screen but only if lcd scaling and
centering are not used as it is currently too dificult to propagate a
different resolution (from what the application thinks) to the correct
output device. Hopefully this does not introduce regressions as the
polarity of non-CRT devices was completly ignored before.

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