drm/i915: Import the kfence selftests for i915_sw_fence

A long time ago, I wrote some selftests for the struct kfence idea. Now
that we have infrastructure in i915/igt for running kselftests, include
some for i915_sw_fence.

v2: INIT_WORK_ONSTACK/destroy_work_on_stack (Mika)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517121007.27224-2-chris@chris-wilson.co.uk