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>

drivers/dma-buf/dma-fence.c

@@ -283,6 +283,7 @@ EXPORT_SYMBOL(dma_fence_begin_signalling);
 
 /**
  * dma_fence_end_signalling - end a critical DMA fence signalling section
+ * @cookie: opaque cookie from dma_fence_begin_signalling()
  *
  * Closes a critical section annotation opened by dma_fence_begin_signalling().
  */