No longer create a work item to clean each execlist queue item.
Instead, move retired execlist requests to a queue and clean up the
items during retire_requests.

v2: Fix legacy ring path broken during overzealous cleanup

v3: Update idle detection to take execlists queue into account

v4: Grab execlist lock when checking queue state

v5: Fix leaking requests by freeing in execlists_retire_requests.

Issue: VIZ-4274
Signed-off-by: NThomas Daniel <thomas.daniel@intel.com>
Reviewed-by: NDeepak S <deepak.s@linux.intel.com>
Reviewed-by: NAkash Goel <akash.goels@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>