msm: ocmem: Use kernel completions instead of wait_event

Migrate to the completion API for DM operationial events
instead of relying on wait_event_*(). wait_event_*() can
return -ERESTARTSYS if a signal is pending instead of waiting
and completions are a convenient abstraction for dealing with
such conditions.

Change-Id: I15352ef23353c505c9f027e847bd673f7a04d925
Signed-off-by: Naveen Ramaraj <nramaraj@codeaurora.org>