msm: camera: Set event data stat buffer to NULL after freeing

Set stats buff event data to NULL after dequeuing & freeing
the event to avoid possibility of freeing the data again.

Change-Id: Iba069ff3d118b276694ebfa04af2b03e454a8583
CRs-Fixed: 463130
Signed-off-by: Raju P.L.S.S.S.N <rplsssn@codeaurora.org>