drm/i915: Show bounds of active request in the ring on GPU hang

Include the position of the active request in the ring, and display that
alongside the current RING registers (on a GPU hang).

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/20161004201132.21801-6-chris@chris-wilson.co.uk