dma-buf/sw_sync: put fence reference from the fence creation

Once sw_sync_ioctl_create_fence() returns we no longer have the
*pt pointer to the fence base object thus we need to put the reference
we have from the fence creation to keep a correct reference accounting.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1477515599-7685-1-git-send-email-gustavo@padovan.org