drm/panel: ld9040: Register a backlight device

Register a backlight device to be able to switch between all the gamma
levels.

v2: Remove .get_brightness() callback, use bl_get_data() and
    backlight_get_brightness()

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230708084027.18352-3-paul@crapouillou.net