u_transfer_helper already had code to handle treating packed Z32_S8
as separate Z32_FLOAT and S8_UINT resources, since some drivers can't
handle that interleaved format natively.

Other hardware needs depth and stencil as separate resources for all
formats.  For example, V3D3 needs this for 24-bit depth as well.

This patch adds a new flag to lower all depth/stencils formats, and
implements support for Z24_UNORM_S8_UINT.  (S8_UINT_Z24_UNORM is left
as an exercise to the reader, preferably someone who has access to a
machine that uses that format.)

Reviewed-by: Eric Anholt <eric@anholt.net>