intel: Track miptrees for fake packed depth/stencil renderbuffers.

intel: Track miptrees for fake packed depth/stencil renderbuffers.

Right now the fake packed d/s RBs are creating two sub-renderbuffers
with their own storage, and the hardware setup and the mapping code
have been explicitly referencing them.  By setting miptrees on them,
we'll be able to make our renderbuffer code for fake packed
depth/stencil more consistent with all our other renderbuffers.

The interesting new behavior here is that there is now a mt with a
non-depthstencil format (X8Z24) that has a stencil_mt field
associated.  This looks like it should be safe, and we'll need to be
able to do this for floating point depth/stencil as well.