drm/panel: nt36672a: add backlight support

Add support for backlight. This panel supports backlight control
through the QCOM WLED driver in Xiaomi Poco F1 device.

Signed-off-by: Joel Selvaraj <jo@jsfamily.in>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/BY5PR02MB700935F5817128CB7C3991CDD9E09@BY5PR02MB7009.namprd02.prod.outlook.com