backlight: lp855x: Move device_config setting out of lp855x_configure()

Move the setting of the lp->cfg pointer to the chip specific
lp855x_device_config struct from lp855x_configure() to
lp855x_probe(), before calling lp855x_parse_dt().

This is a preperation patch for adding ACPI enumeration support.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211102225504.18920-1-hdegoede@redhat.com