viafb: add interface for output device configuration

This patch extends the proc entry to contain a possibility to view and
change the output devices for each IGA. This is useful for debugging
output problems as it provides a reliable way to query which low level
devices are active after VIAs output device configuration nightmare
happended. It's as well suitable for daily use as one can change the
output configuration on the fly for example to connect a projector.
At the moment it's still unstable. The reason is that we have to handle
a bunch of undocumented output devices (those without a proper name) and
that this patch is the first step to collect and verify the needed
information. Basically the only configuration change that is expected to
work at the moment is switching output devices between IGA1 and IGA2.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Andrew Morton <akpm@linux-foundation.org>