drm/i915/selftests: Apply a heavy handed flush to i915_active

Due to the ordering of cmpxchg()/dma_fence_signal() inside node_retire(),
we must also use the xchg() as our primary memory barrier to flush the
outstanding callbacks after expected completion of the i915_active.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306133852.3420322-1-chris@chris-wilson.co.uk