drm/i915/gt: Pull marking vm as closed underneath the vm->mutex

Pull the final atomic_dec of vm->open (marking the vm as closed)
underneath the same vm->mutex as used to close it. This is required to
correctly serialise with attempting to reuse the vma as the vm is closed
by a second thread.

References: 00de702c

 ("drm/i915: Check that the vma hasn't been closed before we insert it")
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/20200227085723.1961649-10-chris@chris-wilson.co.uk