msm: kgsl: Handle fence_add_callback error correctly

The function fence_add_callback returns error if the fence
has already signaled. Handle it correctly in kgsl and do
not propagate this to userspace.

Change-Id: Iabc50b271f1d834c9763eb00d1d296f2b19fbbcd
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>