msm: camera_v2: add locks around session operations.

This change makes sure that all buffer queue manipulation operations
are properly synchronized. It also removes the stream pointer
from the buffer since this memeory can get freed
from underneath without any notification.

CRs-Fixed: 472605 472986
Change-Id: Ie15dd42a02b8bfea1100dca7a7b5ccdbabe488c7
Signed-off-by: Ankit Premrajka <ankitp@codeaurora.org>