drm/i915: use kobj_to_dev()

Use kobj_to_dev() instead of open-coding it.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/f5db8b56cd177972c901d01aa87ba763735438a9.1452696179.git.geliangtang@163.com