viafb: add X server compatibility mode

This patch adds a config option to be compatible with X servers like
OpenChrome. This is required as for example the X server does not
handle things like disabled IGAs/PLLs resulting in a potential
freeze on X startup. With this option disabled we can provide some
nice features like power management and not reinitializing the
hardware on every mode switch (taking long time, causing flickering).

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