qseecom: Fix race condition when loading app

When two clients issue a 'qseecom_start_app' request to load
and start the same app, check_if_app_exist() call may return
an incorrect status of the app to have been already loaded.

Fix is to protect/mutex_lock the entire region that checks
if the app is already loaded and loads the app successfully.

Change-Id: I7a23ae8c8935c7c0c2e7c3c6439742d3e8fe7b42
Signed-off-by: Mona Hossain <mhossain@codeaurora.org>