drm/i915: Mark the shadow gvt context as closed

As the shadow gvt is not user accessible and does not have an associated
vm, we can mark it as closed during its construction. This saves leaking
the internal knowledge of i915_gem_context into gvt/.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161218153724.8439-5-chris@chris-wilson.co.uk