disp: msm: dp: check branch device when adding mst sim port

When adding a new MST sim port check if the branch device was
destroyed already to avoid dereferencing a null pointer.

Change-Id: I8e6f82c5a6772a9b85d24feca52368da2027e96d
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>