dma-fence: Some kerneldoc polish for dma-fence.h

- Switch to inline member docs for dma_fence_ops.
- Mild polish all around.
- hyperlink all the things!

v2: - Remove the various [in] annotations, they seem really uncommon
in kerneldoc and look funny.

v3: Linebreak the "Returns" part of the @fill_driver_data kerneldoc
(Eric).
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180427061724.28497-2-daniel.vetter@ffwll.ch