viafb: add new funcions to select a single mode

This patch introduces 2 new functions for selecting a single mode
based on hres, vres and refresh rate and changes some uses to use
those. The advantage is that it is less error prone than doing the
selection based on refresh rate everywhere and allows replacing the
modetable structure. This includes a little change that users may
notice: If a refresh rate was given as module parameters but does
not exist in the modetable prior to this patch a refresh rate of 60
was assumed and after this patch the closest supported refresh rate
to the one provided by the user is used.

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