drm/panel: otm8009a: Drop unnessary backlight_device_unregister()

It's not necessary to unregister backlight device which
registered with devm_backlight_device_register().

Fixes: 12a6cbd4

 ("drm/panel: otm8009a: Use new backlight API")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200618134650.44311-1-weiyongjun1@huawei.com