drm/i915/gt: Tidy up checking active timelines during retirement

Use the status of the timeline request list as we retire it to determine
if the timeline is still active.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191223150833.2329366-1-chris@chris-wilson.co.uk