viafb: add initial VX900 support

This patch adds basic support for the new VX900 IGP. Almost everything
that was implemented for other IGPs is expected to work also on VX900
after this patch. The only known issue is that on the CRT output mode
setting does not always work.
It is clear that the possibility for regressions is zero.

A big thanks to VIA Technologies for making this possible and
supporting this work.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Jonathan Corbet <corbet@lwn.net>