msm: mdss: add proper locking during suspend/resume operations

Add locking mechanism to properly syncrhonize user space frame updates
with suspend/resume operations. This should avoid some cases in which
suspend operations will complete while frame update hasn't finished,
leading to unwanted behavior.

Change-Id: I4b254a6a547320199a6da722ef99563696686a20
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>