viafb: split pll configs up

This patch splits the pll configs up on pll versions. This allows
easy adding of other known good pll values. Additionally it made it
possible to remove invalid configurations resulting in better
behaviour for such cases. The resulting clocks are no longer stored
resulting in some computing overhead on each mode change.

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