msm: mdss: acquire list_lock before calling free pipe

If unblank call and display thread tries to free pipe
list at the same time; one of the caller might hit the
null free list and leads to crash. Acquire list_lock
before calling the free pipe API.

Change-Id: Ieb6af1700803e3049ea918e2be358e55e43d477a
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>