viafb: do modesetting after updating variables

Reorder viafb_set_par to allow using the updated variables in
viafb_setmode.  This fixes a regression that prevented proper runtime mode
changes.

Signed-off-by: Erik-Jan Post <ej.lfs@xs4all.nl>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Scott Fang <ScottFang@viatech.com.cn>
Cc: Joseph Chan <JosephChan@via.com.tw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>