backlight: rave-sp: Leave initial state and register with correct device

This way the backlight can be referenced through its device node and
enabling/disabling can be managed through the panel driver.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>