lxfb: Program panel v/h sync output polarity correctly

Commit b5c26f97 introduced some breakage for the OLPC XO-1 laptop,
differences in the output video signal after the patch caused some problems
with the XO's display controller chip.

Reviewing of that commit against the AMD Geode LX Data Book, it seems
that these bits were being set inversely. In both cases, active high
output is denoted by a value of 0. See section 6.8.3.44 of the databook
from February 2009 (Publication ID: 33234H)

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>