drm/panel: nt36523: Add DCS backlight support

This chip supports controlling the backlight via DCS commands, on at
least some panels. Add support for doing so.

Note this may only concern the NT36523*W* variant. Nobody knows, really,
there's no docs.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Jianhua Lu <lujianhua000@gmail.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230412-topic-lenovopanel-v3-2-bcf9ba4de46f@linaro.org