Displacement with USD MaterialX Subnet does not work

**Describe the bug**
Displacement using mtlxdisplacement and mtlximage nodes in a subnetwork does not work with Arnold (it does work with Karma)..

**To Reproduce**
Steps to reproduce the behavior:
1. Extract attached archive
2. kick the usda file, or load the hip into Houdini and render with the Arnold render delegate

**Used Software Versions**
- latest Arnold and HtoA

**Additional context**
If I resave the usd as ass, I do something about displacement in the log, but there is nothing visible in the render.
```
00:00:05 400MB | -----------------------------------------------------------------------------------
00:00:05 400MB | displacement
00:00:05 400MB | meshes 1
00:00:05 400MB | shader calls 21025
00:00:05 400MB | -----------------------------------------------------------------------------------
```
[test_displacement_matx.zip](https://github.com/Autodesk/arnold-usd/files/11089778/test_displacement_matx.zip)

Extract test_displacement_matx.zip and reproduce the issue by kicking the USDA file or loading the HIP file in Houdini with the Arnold render delegate. Compare the result with Karma and inspect the Arnold log; done means displacement from the MaterialX subnet is visible in the Arnold render.

Written by the indexing model from the issue text.