tdfxfb: palette fixes

This patch fixes:
- palette setting in 8-bit mode (aka 'funky penguin')
- grayscale handling
- adds proper barriers in xxx_inb/outb functions

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>