sa1100fb: fix color component length for pseudocolor modes

sa1100fb incorrectly sets the length of the color fields to 8 bits for
PSEUDOCOLOR modes for which only 4 bits are used per pixel.  Fix this by
setting the length to 4 bits for these modes.

Signed-off-by: Michal Januszewski <spock@gentoo.org>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>