drm/i915: Flush the i915_vm_release before ggtt shutdown

drm/i915: Flush the i915_vm_release before ggtt shutdown

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: b32fa8111563 ("drm/i915/gtt: Defer address space cleanup to an RCU worker")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]