When doing simple scenes the displacement shading failed during final
rendering when the displacement method is set to Displacement + Bump.

When this option is enabled the shader evaluation uses the Vector math
node. This node is part of the node group level 1. When doing simple
shading where only nodes where used that are part of the node group level
0 the shading was rendered black.

This only happened in final rendering as there the OpenCL programs are
optimized to save registries. Viewport rendering rendered correctly