When using OpenCL with Cycles the rendering time increased substantial.
After doing some tests the bottleneck was found in 4d voronoi and 2d and
3d smooth voronoi.
This change will hide these behind a specific compile directive so the
speed will improve.
AMD RX480 + BMW scene
2.80 (3:10)
2.81 (5:48)
2.81 excluding 4d voronoi+2d/3d smooth (3:50)
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D6231
e527544b76