Add warning for nesting dma_fence containers

[Christian König]

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.