drm/i915: Remove stray intel_engine_cs ring identifiers from i915_gem.c

drm/i915: Remove stray intel_engine_cs ring identifiers from i915_gem.c

A few places we use ring when referring to the struct intel_engine_cs. An
anachronism we are pruning out.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-9-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469606850-28659-4-git-send-email-chris@chris-wilson.co.uk