UPSTREAM: video / backlight: add two APIs for drivers to use

UPSTREAM: video / backlight: add two APIs for drivers to use

It is useful to get the backlight device's pointer and use it to set
backlight in some cases(the following patch will make use of it) so add
the two APIs and export them.

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 f6a4790a5471d7cba406d87f6b41323f40bb93d2)

Change-Id: Ia7d06bf600f86ae25939b830e49f5057f48f992d
Signed-off-by: David Wu <david.wu@rock-chips.com>