fbdev/rivafb: Reorder backlight and framebuffer init/cleanup

The driver's backlight code requires the framebuffer to be
registered. Therefore reorder the init and cleanup calls for
both data structures.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Antonino Daplas <adaplas@gmail.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-24-tzimmermann@suse.de