dsp: check ac before access the service id

Check the audio client and apr before retrieving the
service_id and avoid the race condition b/w q6asm_session_free
and get_service_id calls with mutex lock.

Change-Id: I05c5fb872f866c79c909123b595df345da828967
Signed-off-by: Prasad Kumpatla <nkumpat@codeaurora.org>