matroxfb: color setting fixes

- the pseudo_palette is only 16 elements long.
- do not write to the pseudo_palette if regno (array index) is more than 15
- remove code that writes to the 17th entry of the pseudo_palette

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Acked-by: Petr Vandrovec <vandrove@vc.cvut.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>