During the context-close, we should be decoupling all the vma from the
object so that upon object-closing we shouldn't see any vma from the
already closed contexts. So include a check upon closing the object that
the context is still open.

v2: Eek, the fpriv check is required for shared objects. Double eek, BAT
passed?
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170822110517.22277-1-chris@chris-wilson.co.ukReviewed-by: Michał Winiarski <michal.winiarski@intel.com>