dma-buf: fix shared fence list handling in reservation_object_copy_fences

Add some helpers to correctly allocate/free reservation_object_lists.

Otherwise we might forget to drop dma_fence references on list destruction.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322031/?series=64786&rev=1