drm/msm/dsi: initialize postdiv_lock before use for 10nm pll

postdiv_lock spinlock was used before initialization
for 10nm pll. It causes following spin_bug:
	"BUG: spinlock bad magic on CPU#0".
Initialize spinlock before its usage.

Changes in v3:
- Added Archit's R-b

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rajesh Yadav <ryadav@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>