drm/i915: Set fence_work.ops before dma_fence_init

Since dma_fence_init may call ops (because of a meaningless
trace_dma_fence), we need to set the worker ops prior to that call.

Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Fixes: 8e458fe2

 ("drm/i915: Generalise the clflush dma-worker")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191212154224.1631531-1-chris@chris-wilson.co.uk