drm/i915: Add a few more sanity checks for stolen handling

We should never be called via obj->ops->release() on anything other than
a fully formed stolen object, so raise that to an assert. In the process
tidy up a comment and variable no longer used outside of a conditional
BUG.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161117155846.4631-1-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>