dma-buf: fix kernel-doc warning in dma-fence.c

dma-buf: fix kernel-doc warning in dma-fence.c

Add @cookie to dma_fence_end_signalling() to prevent kernel-doc
warning in drivers/dma-buf/dma-fence.c:

../drivers/dma-buf/dma-fence.c:291: warning: Function parameter or member 'cookie' not described in 'dma_fence_end_signalling'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/388527/
Signed-off-by: Christian König <christian.koenig@amd.com>