Change how amdgpu stores fences in dma_resv objects

Since we can't find a consensus on hot to move forward with the dma_resv object I concentrated on changing the approach for amdgpu first.

This new approach changes how the driver stores the command submission fence in the dma_resv object in DMA-buf exported BOs.

For exported BOs we now store the CS fence in a dma_fence_chain container and assign that one to the exclusive fences slot.

During synchronization this dma_fence_chain container is unpacked again and the containing fences handled individually.

This has a little bit more overhead than the old approach, but it allows for waiting for the exclusive slot for writes again.

Regards, Christian.