msm: mdss: protect retire count with mutex lock

The retire timeline counter in command mode is used
by vsync worker queue, validate/commit ioctl thread
and overlay off. However, this counter is not protected
with any locking mechanism and each thread updates the
value. Race condition with updated value between two
thread can lead to invalid vsync operation or list
corruption. This change protects the counter with
mutex lock to avoid all such race conditions.

Change-Id: Ib3c3f6c839fc82c6d31fa28066df1f23dab19746
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>