drm/cirrus: flip default from 24bpp to 16bpp

drm/cirrus: flip default from 24bpp to 16bpp

The problem with 24bpp is that it is a rather unusual depth these days,
cirrus is pretty much the only relevant device still using that, and it
is a endless source of issues.  Wayland doesn't support it at all.  Bugs
in Xorg keep showing up.

Typically either 32bpp or 16bpp are used.  Using 32bpp would limit the
resolution to 800x600 due to hardware constrains.  So lets go with 16bpp.

Also use the default depth for the framebuffer console and
mode_info->preferred_depth.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180808111311.30311-1-kraxel@redhat.com