viafb: use new device routing

This patch uses the iga{1,2}_devices variables to select which IGA
should be the source. Doing this is convinient, more powerfull
than the older scheme and easy extendable to support further output
devices. It is not yet completed as the device on/off selection needs
to be converted to the same scheme to take full advantage.
No visible changes yet as we want to complete the transition before
anouncing any unstabke interface.

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