hwc: mdpcomp: Free pipeinfo objects every drawing round

Free pipeinfo objects every round on reset, irrespective of the
number of layers to avoid any possible leaks.

Change-Id: Ia47c968f6ee64cbb56be72336305bf11a45c2ee5
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