fbdev: Do not include <linux/backlight.h> in header

Forward declare struct backlight_device and remove the include
statement.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240219093941.3684-5-tzimmermann@suse.de