viafb: update fix before calculating depth

As the depth calculation depends on information in fix it is saner to
do the update first.
No runtime change expected as the value visual in fix used never
changes to MONO.
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>