msm7x30: Enable MDP composition

Change-Id: I6ff5ba6ae07b682d4cd463841678b7aae18a1a31
msm7x30: Fix default USB mode

* Default USB mode is part of default.prop and not build.prop
* Override the property correctly

Change-Id: Ic4c265721b7b859713e6639971443967973f254b