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

drivers/gpu/drm/tiny/cirrus.c

@@ -604,7 +604,7 @@ static int cirrus_pci_probe(struct pci_dev *pdev,
 	if (ret)
 		return ret;
 
-	drm_fbdev_generic_setup(dev, dev->mode_config.preferred_depth);
+	drm_fbdev_generic_setup(dev, 16);
 	return 0;
 }