drm/i915: Generalise the clflush dma-worker

Extract the dma-fence worker used by clflush for wider use, as we
anticipate using workers coupled to dma-fences more frequently.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821191606.17001-1-chris@chris-wilson.co.uk