drm/i915/debugfs: use rb_entry()

To make the code clearer, use rb_entry() instead of container_of() to
deal with rbtree.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/62ce937ae9a341421942b4418515610d055fa653.1482158544.git.geliangtang@gmail.com