fbdev/radeonfb: Use hardware device as backlight parent

Use the hardware device in struct fb_info.device as parent of the
backlight device. Aligns the driver with the rest of the codebase
and prepares fbdev for making struct fb_info.dev optional.

v2:
	* add Cc: tag (Dan)

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/20230613110953.24176-23-tzimmermann@suse.de