msm: mdss: extend mutex offlock for dfps updates

Extend mutex offlock for DFPS to avoid the race condition between dfps
updates and video stop. During dfps update, driver waits for vsync
done interrupt to program registers for new frame rate, however, at
the same time, if display is turned off for suspend, device may
crash. These two operations have to be mutually exclusive.

Change-Id: Ia0c8e037bb4316ec2ee51ac4f9f14463f6ac107a
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>