Subdivision iteration primvar not inherited on instances

**Describe the bug**
When we set the attribute primvars:arnold:subdiv_iterations on a prim, the children inherit this primvar and set the subdiv on the meshes. In Arnold it will only work if the scene in parts of the scene that are not instanced but not on the instanced one.

**To Reproduce**
Steps to reproduce the behavior:
1. Using the pixar kitchen set non instanced, add the attribute at the root prim with a value of 5 for example
2. render with arnold - we can see the subdivision
3. Using the pixar kitchen set instanced, add the attribute at the root prim with a value of 5 for example
4. we can't see the subdivision.

Reproduce the difference with the Pixar kitchen set in non-instanced and instanced forms, setting primvars:arnold:subdiv_iterations to 5 at the root and rendering with Arnold. Trace how inherited primvars are handled for instances; done when the instanced scene shows the same subdivision as the non-instanced scene.

Written by the indexing model from the issue text.