This is used for handling future fences. Currently no driver use
these, and I think given the new timeline fence proposed by KHR it
would be better to have a more abstract interface for future fences.
Could be something simple like a struct dma_future_fence plus a
function to add a callback or wait for the fence to materialize.

Then syncobj (and anything else really) could grow new functions to
expose these two drivers. Normal dma_fence would then keep the nice
guarantee that they will always signal (and through ordering, be
deadlock free). dma_future_fence would then be the tricky one.

This also fixes sphinx complaining about the kerneldoc.

Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180822092905.19884-1-daniel.vetter@ffwll.ch