viafb: use information in var for modesetting

viafb: use information in var for modesetting

This patch starts to use the information in var for modesetting for
CRT and DVI devices. This is the right thing as it allows us to use
more generic modes than the ones predefined by VIA. We do not yet
allow more generic modes as check_var still limits them to the
predefined ones but with this patch applied it would be really easy
to do so. A problem was VIAs SAMM mode as it has 2 different modes
but just one frame buffer device. This is solved by creating a
pseudo var which contains enough information to use it for
modesetting. Hopefully one day we can use information in var for all
modes that do not involve hardware scaling.
Well I'd like to say that the chance of regressions is low but it is
quite likely that the behaviour in some cases changed especially
when SAMM is involved. I hope we made it better than before in
particular the DVI frequency check was probably broken before and
hopefully works better now.

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