Chaining of dma_fence_chain objects is only allowed through the prev
fence and not through the contained fence.

Warn about that when we create a dma_fence_chain.

v2: fix comment style
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220204100429.2049-4-christian.koenig@amd.com