Blender Git Loki

Fix T82336: Cycles standard attributes missing in displacement shaders

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