liboverlay: Invoke commit before freeing the writeback device

MDP releases resources programmed for the WB device only when the
device is not used by anyone else. So freeing up the WB device from
HWC doesn't always guarantee that all the pipes staged on the
device will be freed up. Always invoke the commit explicitly
before releasing WB device to unstage the resources.

Change-Id: I1d98b489eda3baf12449f84edfb3b3f8b84158d9