viafb: remove duplicated scaling code

viafb: remove duplicated scaling code

The code for P4M900 does the same as for all newer IGPs so there is no
reason to duplicate it. Just reducing the code to maintain.

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