tracing/dma-buf/fence: Fix timeline str value on fence_annotate_wait_on

timeline was wrongly assigned with ->get_driver_name().

Link: http://lkml.kernel.org/r/1453376895-30747-1-git-send-email-gustavo@padovan.org

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>