drm/i915: Dump w/a lists on all engines

We store separate wa_list on every engine, so be sure to include all
when dumping the current set via debugfs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703135805.7310-1-chris@chris-wilson.co.uk