drm/i915/selftests: Attach the fence to the object when making busy

make_obj_busy() makes a dummy busy object, but didn't attach the fence
to the reservation object, so it would not have registered as busy. For
completeness, attach the dummy request as the exclusive fence and mark
the object as written (in i915_vma_move_to_active)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180629133717.11761-2-chris@chris-wilson.co.uk