rust: drm/gpuvm: Pass vm_bo explicitly to step_remap()

We cannot drop ARef<GpuVmBo<T>> references within the step_*() calls,
since the destructore takes the object lock but that is already locked
here. Instead of providing a method that the callback can use to obtain
a reference (which, when dropped, would deadlock), grab a reference
ourselves and pass it explicitly into the callback as a &ref. Thus, we
can drop it without locking again.

Signed-off-by: Asahi Lina <lina@asahilina.net>