drm/panel: Use of_find_backlight helper

Replace of_find_backlight_by_node and of the code around it
with of_find_backlight helper to avoid repetition of code.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Meghana Madhyastha <meghana.madhyastha@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/140d01afb138d687680b2d1776a4c101c9fa9a0a.1516810725.git.meghana.madhyastha@gmail.com