display: Add flag to allow legacy allocations from MM heap

 * New devices with IOMMU use the MM heap exclusively for protected
   buffers. We need to support the old behavior for devices with legacy
   userspace.
 * Enable the old behavior with TARGET_DISPLAY_INSECURE_MM_HEAP.

Change-Id: I7882354fa8f159c707be11d7021c18561abfb522