drm/i915/gt: Fix termination condition for freeing all buffer objects

drm/i915/gt: Fix termination condition for freeing all buffer objects

A last minute change, that unfortunately broke CI so badly it declared
SUCCESS, was to refactor the debug free all buffer pool code to reuse
the normal worker, inverted the termination condition so that it instead
of discarding the nodes, they were all declared young enough and
eligible for reuse.

Fixes: 06b73c2d0b65 ("drm/i915/gt: Delay taking the spinlock for grabbing from the buffer pool")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200729110756.2344-1-chris@chris-wilson.co.uk
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[Joonas: Updating Fixes: link after rebasing and reordering into drm-intel-gt-next]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>