dma-buf: further relax reservation_object_add_shared_fence

Other cores don't busy wait any more and we removed the last user of checking
the seqno for changes. Drop updating the number for shared fences altogether.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322379/?series=64837&rev=1