No helper function to do it all yet provided since no driver has
support for driver core fences yet. Which we'd need to make the
implementation really generic.

v2: Clarify async howto a bit per the discussion With Rob Clark.

Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: NSean Paul <seanpaul@chromium.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>