This panel is only visible when debug_value is set to 256 and has no
affect in other cases. However, if debug value is not set to this
value, environment variables will be used to control which features
are enabled, so there's no visible changes to anyone in fact.
There are some changes needed to prevent devices re-enumeration on
every Cycles session create.
Reviewers: juicyfruit, lukasstockner97, dingto, brecht
Reviewed By: lukasstockner97, dingto
Differential Revision: https://developer.blender.org/D1720