drm/i915: Show waiters in i915_hangcheck_info

It is convenient to know what processes are waiting when looking at
hangcheck status in debugfs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161004201132.21801-8-chris@chris-wilson.co.uk