msm: mdss: fix NOC error issue on mdss-pll

mdss-pll register may be accessed when mdss-pll is not enabled.
It is caused by a race condition.
This change is to add a mutex to avoid race condition.

Change-Id: I3b53fd9be55f8de6c234222fe28eba5da2819713
Signed-off-by: Xu,Baochu <bxu@codeaurora.org>