drm/doc: Include fence chain api

We have this nice kerneldoc, but forgot to include it.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210521082457.1656333-1-daniel.vetter@ffwll.ch

Documentation/driver-api/dma-buf.rst

@@ -178,6 +178,15 @@ DMA Fence Array
 .. kernel-doc:: include/linux/dma-fence-array.h
    :internal:
 
+DMA Fence Chain
+~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-fence-chain.c
+   :export:
+
+.. kernel-doc:: include/linux/dma-fence-chain.h
+   :internal:
+
 DMA Fence uABI/Sync File
 ~~~~~~~~~~~~~~~~~~~~~~~~