Graphic: fix mutex starvation issue when changing live wallpaper many times.

Graphic: fix mutex starvation issue when changing live wallpaper many times.

BZ: 26394

PVRSRV_BridgeDispatchKM and PVRDRMUnprivCmd have mutex protection in themselves.
So should use DRM_UNLOCKED.
Wallpaper and surfaceflinger will call drm_ioctl.
If do not use DRM_UNLOCKED, PVRSRV_BridgeDispatchKM will be protected by another mutex.
And it will cause mutex starvation issue.

Change-Id: I060f78db4bfeede32cabe581cd86634ac208091f
Signed-off-by: Pan Zhenjie <zhenjie.pan@intel.com>
Reviewed-on: http://android.intel.com:8080/39710
Reviewed-by: Shi, Yang A <yang.a.shi@intel.com>
Reviewed-by: Xu, Randy <randy.xu@intel.com>
Tested-by: Tong, BoX <box.tong@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>