drm/i915/selftests: Track evict objects explicitly

During review of commit 71fc448c ("drm/i915/selftests: Make evict
tolerant of foreign objects"), Matthew mentioned it would be better if
we explicitly tracked the objects we created. We have an obj->st_link
hook for this purpose, so add the corresponding list of objects and
reduce our loops to only consider our own list.

References: 71fc448c ("drm/i915/selftests: Make evict tolerant of foreign objects")
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/20190121222117.23305-6-chris@chris-wilson.co.uk