viafb: rework color checking

Make color checking a bit more tolerant in what values it allows and more
fine grained to later support 15 and 30 bits formats.  It splits the
filling of the color information in var to a seperate function and sets
some color related values in var that where previously untouched.

This could be a bug fix but at least I don't know any applications that
was fooled by not correctly setting the fields in var.  At least no
regressions are expected.

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>