backlight: lcd: Fix race condition during register

Once device_register is called for a device its attributes might be
accessed. As the callbacks of a lcd device's attributes make use of the
lcd_ops, the respective member must be setup before calling
device_register.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>