viafb: modetable conversion

This patch converts the modetables used in viafb to
- remove the strange thing that sync_end and blanking_end contained
  the length and not the absolute value
- remove hundreds of useless defines
- use fb_videomode and not our own definition so modes defined in
  the subsystem and received via EDID are compatible with ours

As the modes are now stored in a flat structure and no longer in a
tree like thing the lookup time was increased but as it is a rare
event anyway it shouldn't matter. Otherwise the behaviour should be
the same as before.

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