cirrusfb: fix 16bpp modes

The 16bpp mode did not work on the Cirrus cards as the visual type was set
to DIRECTCOLOR instead of TRUECOLOR.  The Alpine family used one incorrect
register setting so this 16bpp modes generated wrong horizontal frequency.

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