hwc: Deduce overlapping region from layers below PTOR layer

Use intersection of PTOR layer and the layers below it to
address usecases like
   * PTOR layer is not peripheral to the layers below it
   * Some layer/layers completely below PTOR layer

Change-Id: Ieb456a60df12cc2f5ad46fbbba2d99a16853278d
1 file changed