gallium/u_transfer_helper: add util functions for doing deinterleaving during map

gallium/u_transfer_helper: add util functions for doing deinterleaving during map

in some cases (e.g., zink and d3d12) we only want to split the depth and stencil
buffers when we're mapping them, and we can handle packed buffers in other cases,
so being able to reuse the u_transfer_helper functionality is still desired but
only if we can preserve the underlying buffer the rest of the time

Kenneth Graunke notes during post-review:
Vulkan reads/copies on packed Z24S8 only return depth, so we need to use separate
Z24 and S8 reads and do packing tricks.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5338>