viafb: split color mode setting up

This patch splits color mode setting up in seperate functions.  Some
hardware initialization that was previously mixed with it is moved to
viafb_setmode.  As are the calls to the newly created function.  This is
yet another little step towards controlling each IGA on its own.

As this patch really aims too mimic the old behaviour no regressions are
expected.  However I noticed that 8bpp (or 6bpp?) seems actually a bit
broken before and after the patch.

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>