sync: add internal refcounting to fences

If a fence is released while a timeline that one of it's pts is on is being
signaled, it is possible for that fence to be deleted before it is signaled.
This patch adds a refcount for internal references such as signaled pt
processing.

Signed-off-by: Erik Gilling <konkers@android.com>
Change-Id: Ie8605e6fd2ac026c207220a03d84e1c1078ec719