drm/i915: Avoid using intel_engine_cs *ring for GPU error capture

Inside the error capture itself, we refer to not only the hardware
engine, its ringbuffer but also the capture state. Finding clear names
for each whilst avoiding mixing ring/intel_engine_cs is tricky. As a
compromise we keep using ering for the error capture.

v2: Use 'ee' locals for struct drm_i915_error_engine

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-8-git-send-email-chris@chris-wilson.co.uk


Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469606850-28659-3-git-send-email-chris@chris-wilson.co.uk