st/mesa: make Z/S drawpix shaders independent of variants, don't use Mesa IR v2

st/mesa: make Z/S drawpix shaders independent of variants, don't use Mesa IR v2

- there is no connection to user fragment shaders, so having these as
  shader variants makes no sense
- don't use Mesa IR, use TGSI
- don't create gl_fragment_program, just create the shader CSO

v2: generate exactly the same shader as before to fix llvmpipe

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Brian Paul <brianp@vmware.com>