drm/panel: ld9040: add backlight Kconfig dependency

The driver now uses the backlight interface, which breaks when that
is disabled:

ld.lld: error: undefined symbol: devm_backlight_device_register

Enforce the necessary Kconfig dependency to avoid this.

Fixes: c2268daa ("drm/panel: ld9040: Register a backlight device")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230724121736.1293270-1-arnd@kernel.org