viafb: fix OLPC DCON refresh rate

This patch fixes a regression introduced by
	fd3cc698
	"viafb: remove duplicated clock storage"
caused by an incosistent mode which pretended to have a higher
refresh rate than it actually had. The wrong refresh rate resulted
in a calculated higher pixclock which the OLPC DCON could not handle.
By reducing the refresh rate to 50Hz we get close to the old
pixclock which makes the OLPC display usable again.
Minor other adjustments are needed as 60Hz is assumed to be a safe
value which is not true for OLPC DCON. This is no problem as we only
support 1200x900 on the OLPC.

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