dma-buf: Add selftests for dma-fence

Exercise the dma-fence API exported to drivers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819095928.32091-2-chris@chris-wilson.co.uk