backlight: lp8788: Drop support for platform data

The backlight driver supports getting passed platform data. However this
isn't used. This allows to remove quite some dead code from the driver
because bl->pdata is always NULL, and so bl->mode is always
LP8788_BL_REGISTER_ONLY.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240329133839.550065-2-u.kleine-koenig@pengutronix.de


Signed-off-by: Lee Jones <lee@kernel.org>