viafb: rework color setting

This is a rewritten version of viafb_setcolreg.  The hardware register
writes were split up and moved to hw.c where they belong as this is really
low level stuff.  It was made dual fb aware.

Furthermore viafb_setcmap was removed as the problem with 8bpp originated
from a bug in writing multiple color registers at once.  The removal of
viafb_setcmap might introduce a small performance regression but its
certainly better to receive the correct result a bit slower than a garbled
picture fast.  It should give us a working 8bpp mode and is more
extensible than the old hardcoded code.  No other regressions are expected
but as the hardware might be a bit picky it might cause some regressions
in 8bpp mode on some hardware although I doubt that.

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