viafb: add lcd scaling support for some IGPs

viafb: add lcd scaling support for some IGPs

These IGPs should also support lcd scaling but likely this switch was
missed when adding support for them. Fix it, allowing lcd scaling on
CN750, VX800 and VX855. At least this improves the situation for
VX855. (there seems to be another scaling unrelated bug somewhere)

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