drm/i915/selftests: Make evict tolerant of foreign objects

The evict selftests presumed that all objects in use had been allocated
by itself. This is a dubious claim and so instead of asserting complete
control over the object lists, take (temporary) ownership of them
instead.

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/20190118113632.7056-1-chris@chris-wilson.co.uk