fbdev/radeonfb: Reorder backlight and framebuffer cleanup

The driver's backlight code requires the framebuffer to be
registered. Therefore reorder the cleanup calls for both data
structures. The init calls are already in the correct order.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-22-tzimmermann@suse.de