drm/sysfs: use kobj_to_dev()

Use kobj_to_dev() instead of open-coding it.

Link: http://patchwork.freedesktop.org/patch/msgid/3fea991541fbfc4ffece2c174adeb02cb9436c90.1452696179.git.geliangtang@163.com


Signed-off-by: Geliang Tang <geliangtang@163.com>