llvmpipe: fix race between draw and setting fragment shader.

There is a race with u_blitter shaders + pipeline shaders (aaline/aapoint)
where the draw bind can cause a pipeline flush which can use bind_fs_state to
 be reenters and llvmpipe->fs gets the wrong value. Fix this by only
setting the llvmpipe->fs value after the draw binding is complete.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4122>

src/gallium/drivers/llvmpipe/lp_state_fs.c

@@ -3071,14 +3071,14 @@ static void
 llvmpipe_bind_fs_state(struct pipe_context *pipe, void *fs)
 {
    struct llvmpipe_context *llvmpipe = llvmpipe_context(pipe);
-
-   if (llvmpipe->fs == fs)
+   struct lp_fragment_shader *lp_fs = (struct lp_fragment_shader *)fs;
+   if (llvmpipe->fs == lp_fs)
       return;
 
-   llvmpipe->fs = (struct lp_fragment_shader *) fs;
-
    draw_bind_fragment_shader(llvmpipe->draw,
-                             (llvmpipe->fs ? llvmpipe->fs->draw_data : NULL));
+                             (lp_fs ? lp_fs->draw_data : NULL));
+
+   llvmpipe->fs = lp_fs;
 
    llvmpipe->dirty |= LP_NEW_FS;
 }