sync_file: get rid of internal reference count.

sync_file uses the reference count of the file, the internal
kref was never getting moved past 1.

We can reintroduce this if we decide we need it later.

[airlied: fix buildbot warnings]

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170413014144.637-2-airlied@gmail.com