Displacement and Background kernels are selectively used, but always compiled. This patch will not compile these kernels when they are not needed.
Displacement kernel is only used for true displacement.
Background kernel is only used when there is a (Cycles)Light of type `LIGHT_BACKGROUND`.
Reviewed By: brecht, #cycles
Tags: #cycles
Maniphest Tasks: T61971
Differential Revision: https://developer.blender.org/D4412
dabe5cd31a