viafb: fill xres and yres

This patch fills xres and yres in var which is required when a
temporary var is genereated when viafb_SAMM_ON is set and
viafb_dual_fb is not set. It also removes an obsolete comment.

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