msm: vidc: Fix race condition during concurrent sessions

The core->instances list structure is guarded by
mutex core->lock, whenever a new video instance is added
or deleted from the list. However this list was accessed
from certain functions without acquiring the lock, which
resulted in race condition, causing the device to crash.
Also clean up the core locking mechanism.

Change-Id: I6f431bc381d54948611783e19fac853c83a87203
Signed-off-by: Arun Menon <avmenon@codeaurora.org>