msm: synx: Finish signaling and callback dispatch before releasing lock

Finishing the callback dispatch and external fence signaling
within the same atomic lock context prevents the probable race
condition when the dma fence signaling has woken the wait process
which might continue to release the fence once completed before
the actual signaling thread is context switched back. This will
result null pointer dereference as metadata have been cleaned.
Ensuring all the signaling functionalities have completed before
releasing the lock prevent this.
Have also added couple more checks for null pointer dereference.

Change-Id: Ib0ba849dc17dff038a1370efcd268bab8663123f
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>