viafb: Automatic OLPC XO-1.5 configuration

Currently, a long set of viafb options are needed to get the XO-1.5
laptop to output video (there is only 1 configuration that works, that
can't really be autodetected).

This patch automatically detects and configures viafb for the XO-1.5
laptop, meaning all that is required for working display is that
viafb is loaded.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>