drm/panel: otm8009a: Use new backlight API

The backlight API provides new functions to enable and disable the
backlight and which hide the intricacies of achieving the correct
result.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423141054.13128-5-philippe.cornu@st.com