drm/i915: Claim vma while under closed_lock in i915_vma_parked()

Remove the vma we wish to destroy from the gt->closed_list to avoid
having two i915_vma_parked() try and free it.

Fixes: aa5e4453 ("drm/i915/gem: Try to flush pending unbind events")
References: 2850748e

 ("drm/i915: Pull i915_vma_pin under the vm->mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205214159.829727-1-chris@chris-wilson.co.uk