UPSTREAM: video / backlight: remove the backlight_device_registered API

UPSTREAM: video / backlight: remove the backlight_device_registered API

Since we will need the backlight_device_get_by_type API, we can use it
instead of the backlight_device_registered API whenever necessary so
remove the backlight_device_registered API.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 01c3664de62f89f6777e59173ad8e20b5a4c267f)

Change-Id: I12fdee74963c2cbc5b950019598ec218de1b7b5d
Signed-off-by: David Wu <david.wu@rock-chips.com>