Use the backlight support in drm_panel to simplify the driver.
While touching the include files sort them
and divide them up in blocks.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: "Guido Günther" <agx@sigxcpu.org>
Cc: Purism Kernel Team <kernel@puri.sm>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207140353.23967-18-sam@ravnborg.org