msm: vidc: Fix a race condition with core instances list

All list operations for core instances list needs to be
protected by mutex. Add mutex for the same.While adding
the new instance to the core instances list, the list
needs to be protected by mutex. This is protected by
incorrect mutex.Fix an issue to acquire proper mutex.

Change-Id: Ibbb11678881fb5d63ce4161fcb4fcd317ddf3259
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>