viafb: remove unused max_hres/vres

This patch removes the max_hres and max_vres which are not used at
the moment. In general they could be useful but it would be better
to get them via any standard EDID implementation and not the buggy
incomplete one currently used which is also removed as far as
possible.

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