video: amba-clcd: use devm_of_find_backlight

video: amba-clcd: use devm_of_find_backlight

Look up backlight device using devm_of_find_backlight().
This simplifies the code and prevents us from hardcoding
the node name in the driver.

v2:
  - Added Cc: Peter Ujfalusi

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200517190139.740249-2-sam@ravnborg.org