hwc: Correct Display Commit sequence during WFD-HDMI interaction.

When WFD is active and HDMI is plugged in, the pipes allocated
to WFD(mixer 2) are unset. Call Overlay-commit on mixer 2 to
unstage the pipes correctly. This also makes sure that a secure
WFD session is teared down correctly.

Change-Id: I7531339148e400727549decbde72096f2a7dd798