When you unlock the bootloader of your Xperia all DRM keys are automatically deleted and you lose your warranty, see Sony Developer world website. In order to retain your warranty you should be able to reset your phone back to original factory conditions including the DRM keys. The DRM keys are stored on the so-called TA partition. To backup this partition you need to make your phone run Android Marshmallow. Marshmallow is susceptible to Dirtycow vulnerability which makes it possible to create a backup of your TA partition.