GFX-DISPLAY: Add mutex in __dpi_panel_power_off to pretect pipe disabled after DRM_WA...

GFX-DISPLAY: Add mutex in __dpi_panel_power_off to pretect pipe disabled after DRM_WAIT_ON signaled

BZ: 37139

Issue:
On Lexington phones, press power button when playing video, sometimes sounds of video will still
playing for a while.

Solution:
Add mutex in __dpi_panel_power_off to pretect pipe disabled after DRM_WAIT_ON signaled by last
vblank event when playing video, otherwise the last vblank event will lost when pipe disabled
before vblank interrupt coming sometimes, it will cause the media pausing action blocking for 3s.

Change-Id: I2e55e3220619bdacd9cca83c67fbaac6be1dbaa6
Signed-off-by: Li Zhuangzhi <zhuangzhi.li@intel.com>
Reviewed-on: http://android.intel.com:8080/51376
Reviewed-by: Xu, Randy <randy.xu@intel.com>
Tested-by: Tong, BoX <box.tong@intel.com>
Reviewed-by: Ai, Ke <ke.ai@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>