drm/omap: display: 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.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Zheng Bin <zhengbin13@huawei.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Enrico Weigelt <info@metux.net>
Cc: Allison Randal <allison@lohutok.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514191001.457441-2-sam@ravnborg.org