viafb: reduce viafb_set_iga_path usage

The result of viafb_set_iga_path can change after init only in the
hotplug ioctl. So we can move it there rather than doing it always
when a new mode is set. The setup during init is done via a call
from the init chip funtion.
This change enables a stable mapping between the old device scheme
and a new more powerfull one.

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