viafb: video address setting revisited

Set always the correct video address.  Especially do panning right on
multiple IGAs.

This should have no effect on single monitor mode (no SAMM, no dual fb).
For SAMM without dual fb this might break something as I really cannot
image what we are supposed to do for different resolutions with a single
framebuffer as we can't get data out of nowhere (no, they are not set up
in something one would call "expanded").  Previously I got for that funny
colored pictures as the second IGA pointed to video memory that was never
written to.  After the patch it'll work as cloning if the first and second
mode are identical (this was working already without SAMM).  Finally for
dual fb this should push us a step in the right direction.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Scott Fang <ScottFang@viatech.com.cn>
Cc: Joseph Chan <JosephChan@via.com.tw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>