backlight: lp855x: Refactor DT parsing code

This patch refactors the dt parsing code to avoid setting platform_data,
instead just setting lp->pdata directly. This facilitates easier
probe deferral since the current scheme would require us to clear out
dev->platform_data before deferring.

Cc: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Milo Kim <milo.kim@ti.com>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>