drm/i915/selftests: Flush all active callbacks

Flushing the outer i915_active is not enough, as we need the barrier to
be applied across all the active dma_fence callbacks. So we must
serialise with each outstanding fence.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112096


References: f79520bb ("drm/i915/selftests: Synchronize checking active status with retirement")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101181022.25633-1-chris@chris-wilson.co.uk