drm/i915: Show RING registers through debugfs

Knowing where the RINGs are pointing is extremely useful in diagnosing
if the engines are executing the ringbuffers you expect - and igt may be
suppressing the usual method of looking in the GPU error state.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161004201132.21801-7-chris@chris-wilson.co.uk