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_program.h

@@ -59,8 +59,6 @@ struct st_fp_variant_key
    GLuint drawpixels:1;           /**< glDrawPixels variant */
    GLuint scaleAndBias:1;         /**< glDrawPixels w/ scale and/or bias? */
    GLuint pixelMaps:1;            /**< glDrawPixels w/ pixel lookup map? */
-   GLuint drawpixels_z:1;         /**< glDrawPixels(GL_DEPTH) */
-   GLuint drawpixels_stencil:1;   /**< glDrawPixels(GL_STENCIL) */
 
    /** for ARB_color_buffer_float */
    GLuint clamp_color:1;