drm/panel: novatek,nt39016: Reorder calls in probe

The drm_panel_of_backlight() function must be called after
drm_panel_init(), according to the function's documentation; otherwise
the backlight won't be properly initialized.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200820121256.32037-2-paul@crapouillou.net