iris: Fix fence leak in iris_fence_flush

Documentation for pipe_context::flush states:
 "NOTE: use screen->fence_reference() (or equivalent) to transfer
  new fence ref to **fence, to ensure that previous fence is unref'd"

Hence we need to unref previous out_fence.

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/gallium/drivers/iris/iris_fence.c

@@ -184,6 +184,8 @@ iris_fence_flush(struct pipe_context *ctx,
       iris_syncpt_reference(screen, &fence->syncpt[fence->count++],
                             ice->batches[b].last_syncpt);
    }
+
+   iris_fence_reference(ctx->screen, out_fence, NULL);
    *out_fence = fence;
 }