Previous code did associate fence to bo before the fence was emited
and it also didn't lock protected access to ttm sync_obj member.
Both of this flaw leads to possible race between different code
path. This patch fix this by associating fence only once the fence
is emitted and properly lock protect access to sync_obj member of
ttm.

Fix:
https://bugs.freedesktop.org/show_bug.cgi?id=26438
and likely similar others bugs
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>