backlight: lp855x: use devm_backlight_device_register()

Use devm_backlight_device_register() to make cleanup paths simpler.  Also,
unnecessary lp855x_backlight_unregister() is removed.

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