drm/panel-sony-acx424akp: Modernize backlight handling

This converts the internal backlight in the Sony ACX424AKP
driver to do it the canonical way:

- Assign the panel->backlight during probe.
- Let the panel framework handle the backlight.
- Make the backlight .set_brightness() turn the backlight
  off completely if blank.
- Fix some dev_err_probe() use cases along the way.

Tested on the U8500 HREF520 reference design.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210715092808.1100106-1-linus.walleij@linaro.org