I don't think it's only that.we'll see in the coming days if Canonical tries to mainline their Mir back-end itself for Mesa or just simply is trying to reduce the maintenance workload in supporting the out-of-tree code by pushing forward some code (i.e. today's patches)
The Mir backend lives in a part of Mesa that doesn't change much, whereas the DRIimage v7 code (the patches here) lives in a part that can change more. It makes more sense to merge it (and avoid other programers to write again this code, which is partially needed for DRI3 support).
We need this for Prime on Wayland, and for writing glamor dri3 helpers. And RAOF also published again more complete patches, because I asked him, telling him the needs of glamor (the __DRiimage v6 bits were missing from previous patches, and glamor would need them, so he added them) and Prime on Wayland.