Add a `use_gpu()` function to the UI code for Cycles.
This is done to clean up some of the other code (`use_{backend}()`)
and to help isolate `use_multi_device` and `show_device_active` from
their context making them more robust to changes made in other parts
of the UI code.
Pull Request: https://projects.blender.org/blender/blender/pulls/124134
9d1e613292