viafb: merge the remaining output path with enable functions

This patch merges the remaining functionality of the output path
function in the associated enabling functions. This is very natural as
most of the remaining code does actually enable the device.
Just some more or less intelligent code merge. If no stupid mistakes
occured there should be no regressions.

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