u_transfer_helper: Handle Z24X8 for drivers that don't use the interleaved transfer_map

Fixes dEQP-GLES3.functional.texture.format.sized.2d.depth_component24_pot on Asahi, which wants to emulates Z24 as Z32 and use true separate stencil. So Z24S8 becomes a Z32F resource with an S8 separate_stencil glued to it.

Draft because I don't understand why u_transfer_helper has an interleaved path and a non interleaved path and whether I'm supposed to be using the interleaved path (it broke dEQP-GLES3.functional.texture.format.sized.2d.depth32f_stencil8_pot), and because this has a good chance of breaking Zink.

Cc @airlied @zmike