qseecom: Check if app exists before loading the app

There is a chance of send a request to QSEE to load an app that
is already previously loaded. Requesting QSEE to load an app
that is already loaded will result in a failure.

Issuing a request to QSEE to check if app already exists before
issuing a request to load it, all within the same app_access_lock
mutex, fixes this issue.

CRs-Fixed: 420473
Change-Id: I5dfb02c7b0f0144d142fc70084ec1e096ada0439
Signed-off-by: Mona Hossain <mhossain@codeaurora.org>