viafb: add new output device management

This patch adds a new output device management that stores for each
IGA which output devices are routed to it and a compatiblity layer
that converts the old per-output device values in the new format.
Bounding the output devices to each IGA is a central idea of the
cleanup. Doing it this way should be easier and make much more sense
than the old format which happily mixed different output devices
together and did not even take into account that some devices are no
longer available on newer chipsets.

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