msm: mdss: add proper locking for buffer lists

Buffer lists are protected by list_lock to prevent multiple threads from
modifying the list. Locks are missing in a few places which may lead to
potential concurrent access and lead to corruption. Add proper locking
to prevent this.

CRs-Fixed: 806684
Change-Id: I95c5ebbb5cd9c11c38ea9be0826f3b8748632a5b
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>