msm: mdss: fix incorrect mutex unlocking during NOTIFY_UPDATE_STOP

When servicing NOTIFY_UPDATE_STOP command, if the notify object
was uninitialized then mutex lock is obtained on one object but
unlock is done on a different object. This change fixes it.

CRs-Fixed: 1088059
Change-Id: I66cbe48b7f4910228a6af57610a8427fea7fd1f2
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>