msm: mdss: Fix race condition during device shutdown

During device shutdown or restart, two separate contexts can call
mdss_fb_release. One system shutdown/restart thread which calls
shutdown routine registered through device driver. Other closing
of frame buffer devices from user processes. Fix race condition
between these two contexts by using proper locking mechanism and
releasing all the resources.

Change-Id: I583b0f1273d984215e276daf3b6307f3b6e9e8c6
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>