cirrusfb: use 5-6-5 RGB for 16bpp mode

Use the 5-6-5 RGB mode instead of the 5-5-5 mode at 16bpp depth.

It fixes colors in the 16bpp modes on Cirrus Laguna chips.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Geert Uytterhoeven <geert.uytterhoeven@gmail.com>
Cc: Arthur Marsh <arthur.marsh@internode.on.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>