drm/cirrus: correct register values for 16bpp

When the mode is set with 16bpp on QEMU, the output gets totally broken.
The culprit is the bogus register values set for 16bpp, which was likely
copied from from a wrong place.

Addresses https://bugzilla.novell.com/show_bug.cgi?id=799216



Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: David Airlie <airlied@linux.ie>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>