viafb: get rid of the remaining modetable structure assumptions

This patch removes the remaining places where assumptions about the
structure of the modetable were made. Aside from some places where
assumptions are made that certain modes are in the modetable the
only code dealing with the modetable and not just a single mode is
in viamode. This will allow chaniging the modetable and use other
sources for videomodes like the subsystem or EDID.

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