viafb: some small cleanup for global variables

viafb: some small cleanup for global variables

We do not need viafb_second{,_virtual}_{xres,yres} outside of
viafbdev.c so move them there and eliminate the virtual ones where
the only sane usage is done during initalization.

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