backlight: add devm_backlight_device_{register,unregister}()

These functions allow the driver core to automatically clean up any
allocation made by backlight drivers.  Thus it simplifies the error
paths.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>