viafb: enable second display channel at central place

Move the enabling of the second display channel to the generic
output routing function and do it exactly if something is using it.
This unifies a zoo of variants how to do it and does no longer
disable it if LCD is not on the second display which is much saner
as there can be other users who need it. Probably this did not cause
any problems so far as we only recently started dual fb support and
otherwise there won't be other users and LCD is preferred assigned to
second display channel.

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