dma-buf: add dma_fence_chain_contained helper

It's a reoccurring pattern that we need to extract the fence
from a dma_fence_chain object. Add a helper for this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220204100429.2049-6-christian.koenig@amd.com