viafb: Add 1200x900 DCON/LCD panel modes for OLPC XO-1.5

[jc: extensive merge conflict fixes]
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Signed-off-by: Chris Ball <cjb@laptop.org>