drm/panel: novatek,nt39016: Handle backlight the standard way

Instead of manipulating the backlight directly in this driver, register
it in the probe using drm_panel_of_backlight() and let the drm_panel
framework code handle it.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200811002240.55194-2-paul@crapouillou.net