Standard attributes are not added to the attributes requests when
shaders only have displacement. This is because nodes are only
considering the case when the surface socket is connected.
To support this, added `Shader.has_surface_link()` which checks for both
cases (`has_surface` and `has_displacement`) and replaces all checks on
`Shader.has_surface`.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D12240
60d6333b80