drm/panel: sitronix-st7701: use drm_panel backlight support

Use the backlight support in drm_panel to simplify the driver

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207140353.23967-23-sam@ravnborg.org