drm/cirrus: Decouple fbdev bpp value from color depth

Cirrus has a preferred color depth of 16 bit; also use it as fbdev
bpp value. Don't use the color depth directly. It has a different
meaning than bpp and both cannot be used interchangeably.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123115348.2521-4-tzimmermann@suse.de