viafb: reduce OLPC refresh a bit

When allowing some PLL calculation we get a frequency that seems to
be a bit higher than what the OLPC DCON likes resulting in a still
readable but not so good image. We don't really know whether this is
a problem with the calculation formula or the OLPC but as other
displays seem to be happy with the other modes adjusting the OLPC
refresh looks like the better thing. This patch prevents a
regression when dynamic PLL calculation is allowed.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>