Add warning for nesting dma_fence containers

Hi everyone,

Since some operations can then lead to recursive handling nesting dma_fence containers into each other is only allowed under some restrictions.

dma_fence_array containers can be attached to dma_fence_chain containers and dma_fence_chain containers by chaining them together.

In all other cases the individual fences should be extracted with the appropriate iterators and added to the new containers individually.

I've separated the i915 cleanup from this change since it is generally a different functionality and the build bots complained about some issues with those patches.

Most patches are already reviewd, but especially the first one still needs an rb tag.

Please review and comment, Christian.