Reduce usage of the name 'ring' for engines et al

Dave Gordon
2016-07-20T17:16:15Z

Chris Wilson is trying to convert 'ringbuffer' to 'ring', but at present
there's rather too much legacy code using 'ring' for various other things,
usually engines or engine-ids. This patchset converts some of them (but
not as yet the gpu_error or trace code).

Chris: what is your prefered name for a local holding an engine id?
'engine_id' is obvious, but seems overly long and clunky. Anything better?