viafb: limit LCD code impact

This patch cleans the LCD code up. It forbids overwritting global
decissions like what IGA should be used as the source and whether to
enable the second display channel. Additionally it removes a bit code
duplication.

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