drm/i915/breadcrumbs: s/container_of/rb_entry/

In keeping with commit f802cf7e

 ("drm/i915/debugfs: use
rb_entry()"), convert the primary user of the rbtrees over to using
rb_entry rather than the equivalent container_of.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161220104003.8044-1-chris@chris-wilson.co.uk
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>