sync: update new fence status with sync_fence_signal_pt

If a fence's pt is signaled before sync_fence_create is called, the fence
will never transition into the signaled state.  This also address a tiny
race if a merged fence's pt after sync_fence_get_status checks it's status
and before fence->status is updated.

Signed-off-by: Erik Gilling <konkers@android.com>
Change-Id: Ic8e292a323db26c6f04cb4757d920278b3125ff6