The increased amount of BSDF code from Principled BSDF v2 and the
microfacet BSDF led to a big performance regression on Metal and AMD.
We have not been able to find a good workaround for all scenes.
This change disables the Principled Hair BSDF code when it is not used
in the scene. This makes common benchmark scenes faster, but
performance is still bad in scenes that do use it.
Ref #112596
Pull Request: https://projects.blender.org/blender/blender/pulls/113904
e11f031d62