dma-buf/sync-file: fix warning about fence containers

The dma_fence_chain containers can show up in sync_files as well resulting in
warnings that those can't be added to dma_fence_array containers when merging
multiple sync_files together.

Solve this by using the dma_fence_unwrap iterator to deep dive into the
contained fences and then add those flatten out into a dma_fence_array.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220311110244.1245-2-christian.koenig@amd.com