gallium: Make PIPE_CAP_DEPTH_CLIP_DISABLE a tri-state value and use it

Use value "2" to signal that lowering is needed and supported and enable
it accordingly.

v2: - Note in CAP description that this lowering currently requires TGSI
    - use "true" instead of GL_TRUE (both Erik)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_context.c

@@ -699,6 +699,9 @@ st_create_context_priv(struct gl_context *ctx, struct pipe_context *pipe,
       }
    }
 
+   if (screen->get_param(screen, PIPE_CAP_DEPTH_CLIP_DISABLE) == 2)
+      st->clamp_frag_depth_in_shader = true;
+
    /* called after _mesa_create_context/_mesa_init_point, fix default user
     * settable max point size up
     */