viafb: use proper pci config API

This patch alters viafb to use the proper Linux in-kernel API to access
PCI configuration space, rather than poking at I/O ports by itself.

Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Signed-off-by: Harald Welte <HaraldWelte@viatech.com>