msm: vidc: Add mutex for FW load calculation

When driver is calculating the load it should be
protected by mutex. While this calculation is in
progress if new session created then the load
calcuation goes wrong. So this should be protected
by mutex.

Change-Id: I22dd526b1b07d9edd7d7211f63485b2059cbfe57
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>