liboverlay: underp NO_IOMMU case

Fixes memory allocations on chips without IOMMU
Based on https://github.com/CyanogenMod/android_hardware_qcom_display-caf/commit/efa2483261a7d2db82c49e33997f232138999478

Change-Id: Ia2c08e43c49a84d70564ca29717efb6bfc168b88