dma-buf: Prettify typecasts for dma-fence-chain

Inside dma-fence-chain, we use a cmpxchg on an RCU-protected pointer. To
avoid the sparse warning for using the RCU pointer directly, we have to
cast away the __rcu annotation. However, we don't need to use void*
everywhere and can stick to the dma_fence*.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200409110101.18400-1-chris@chris-wilson.co.uk