power: fg-alg: Fix race condition in Time To Empty (TTE)

There is a possible race condition in TTE where it might cause
Out of Bound array access because the buffer gets cleared.
Fix this by adding a lock to TTE.

Change-Id: I2074c42aac9655967a8f77a84362507c3954cf20
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>