viafb: use more compact modesetting functions

This patch replaces the old timing setup code with a redesigned one.
The new code might be slightly faster as it has no conditinals and
does not write the same register multiple times. Also it makes the
comparison to the documentation easier.
Regressions are unlikely but could happen as a lot of hardware is
undocumented.

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