s3fb: fix color component length for pseudocolor modes

s3fb incorrectly sets the length of the color fields to 6 bits for
PSEUDOCOLOR modes, even though 8 or 4 bits are used per pixel.  Fix this
by setting the length to 8 or 4, respectively.

Signed-off-by: Michal Januszewski <spock@gentoo.org>
Acked-by: Ondrej Zajicek <santiago@crfreenet.org>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>