UPSTREAM: 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.

Change-Id: Ib94b7f26893ca39ee3c3a58c335d30a7787facc2
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


(cherry picked from commit 18f5fad2)
Bug: 394776841
Signed-off-by: T.J. Mercier <tjmercier@google.com>