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>

src/mesa/state_tracker/st_cb_drawpixels.h

@@ -45,10 +45,4 @@ st_make_drawpix_fragment_program(struct st_context *st,
                                  struct gl_fragment_program *fpIn,
                                  struct gl_fragment_program **fpOut);
 
-extern struct gl_fragment_program *
-st_make_drawpix_z_stencil_program(struct st_context *st,
-                                  GLboolean write_depth,
-                                  GLboolean write_stencil);
-
-
 #endif /* ST_CB_DRAWPIXELS_H */