drm/i915/selftests: Test i915_sw_fence/dma_fence interop

drm/i915/selftests: Test i915_sw_fence/dma_fence interop

Check that we can successfully wait upon a dma_fence using the
i915_sw_fence, including the optional timeout mechanism.

v2: Account for the rounding up of the timeout to the next second.
Unfortunately, the minimum delay is then 1 second.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115204348.8480-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>