gallium: pipe loader cleanup
The goal is to cut down pipe loader's driver-specific code to make refactoring pipe loader interfaces (in particular the driconf xml) more tractable.
Tagging clover because this changes it but we don't have any CI. I haven't managed to test that the driconf UI still works because it doesn't work for me on master anyway ("Could not detect any configurable direct-rendering capable devices. DRIconf will be started in expert mode").