drm/i915: Rename intel_context[engine].ringbuf

Perform s/ringbuf/ring/ on the context struct for consistency with the
ring/engine split.

v2: Kill an outdated error_ringbuf label

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-14-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-3-git-send-email-chris@chris-wilson.co.uk