As the gen6_ppgtt may refer back to the GGTT for their page-directory
slots, make sure those __i915_vm_release are completed prior to shutting
down the GGTT.

Fixes: b32fa811 ("drm/i915/gtt: Defer address space cleanup to an RCU worker")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190729132412.23380-1-chris@chris-wilson.co.uk