msm: BA: Fix race condition with v4l2 event handler

The v4l2 event handler struct should be initialized
before added to the dev_ctxt->instances list.
The v4l2 event handler struct should be de-initialized
after we remove inst from the core->instances list.

Change-Id: I1faf6bab4232bbcd2ca567a9a11b0c8faa2f45ce
Signed-off-by: Domi Papoi <dpapoi@codeaurora.org>