dma-fence: remove fill_driver_data callback

Noticed while I was typing docs. Entirely unused.

v2: Remove reference in @timeline_value_str too. While at it clarify
why timeline_value_str has a fence parameter - we don't have an
explicit timeline structure unfortunately.

Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Christian König <christian.koenig@amd.com> (v1)
Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: Christian König <christian.koenig@amd.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180502082359.30345-1-daniel.vetter@ffwll.ch