drm/i915/gt: Schedule next retirement worker first

As we may park the gt during request retirement, we may cancel the
retirement worker only to then program the delayed worker once more.

If we schedule the next delayed retirement worker first, if we then park
the gt, the work will remain cancelled.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119162559.3313003-2-chris@chris-wilson.co.uk