drm/i915: Show the execlist queue in debugfs/i915_engine_info

When looking at freezes whilst working on execlists, knowing the order
of the pending requests in the driver is useful.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161027000348.4641-2-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>