drm/i915: Rename residual ringbuf parameters

Now that we have a clear ring/engine split and a struct intel_ring, we
no longer need the stopgap ringbuf names.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-16-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-5-git-send-email-chris@chris-wilson.co.uk