sync: protect unlocked access to fence status

Fence status is checked outside of locks in both sync_fence_wait and
sync_fence_poll.  This patch adds propper barrier protection in these
cases to avoid seeing stale status.

Change-Id: I9d8b6ce6accb415e797df58068a1ccd54e6be445
Signed-off-by: Erik Gilling <konkers@android.com>