drm/i915/selftests: Fill in a little more of the dummy fence

Initialise the dma_fence innards in preparation for making
dma_fence_signal() always check the callback list.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708113038.19251-1-chris@chris-wilson.co.uk