dma-buf: Exercise dma-fence-chain under selftests

A few very simple testcases to exercise the dma-fence-chain API.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200409110101.18400-3-chris@chris-wilson.co.uk