st/dri: always allocate private depth-stencil buffers

st/dri: always allocate private depth-stencil buffers

This disables DRI2 sharing of zbuffers. The window zbuffer is allocated just
like any other texture - through resource_create.

The idea of allocating a zbuffer through DRI2 isn't very useful with MSAA,
where a single-sample zbuffer is useless.

IIRC, the Intel driver does the same thing.

Reviewed-by: Brian Paul <brianp@vmware.com>