surfaceflinger: Allow MTK platforms to render level-3 DRM with the GPU

This allows mediatek devices to specify a property that lets devices
provisioned with level-3 DRM to render protected layers using the
GPU if the persist.gralloc.cp.level3 property is set to 1.

Change-Id: Id922e8c1ff13cc65af179588e8294a1a3483e975