msm: mdss: fix race condition between shutdown thread and fb ioctl

Currently all entires into fb driver are synchronized with shutdown
context except fb ioctl context. fb ioctl is an asynchronous entry to
fb driver which can be called simultaneously from two different user
processes. Synchronize these fb ioctls and shutdown context to prevent
race condition.

Change-Id: I81ce48159fce69caabc3336d70df2bd15c4ee44b
CRs-Fixed: 640199
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>