llvmpipe: Don't advertise S8_UNORM (with feeble attempt at supporting it).

llvmpipe: Don't advertise S8_UNORM (with feeble attempt at supporting it).

S8_UNORM was inadvertedly supported together with Z16_UNORM.

I tried to update the code to accomodate stencil-only -- it seemed a simple
thing to do -- but "fbo-stencil clear GL_STENCIL_INDEX8" still fails,
and it's not worth debugging.

Therefore although this change tries to update for S8_UNORM, it also
disables it completely.

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