viafb: Unmap the frame buffer on initialization error

This was part of Harald's "make viafb a first-class citizen using
pci_driver" patch, but somehow got dropped when that patch went into
mainline.

Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Signed-off-by: Jonathan Corbet <corbet@lwn.net>