freedreno: Fix user const buffer dirtiness

If we went the upload_user_buffer() path, cb->buffer would be null,
causing fd_dirty_shader_resource() to be a no-op.  What we want to
use is &so->cb[index].

Fixes: b43e5aec0d ("freedreno/batch: Move submit bo tracking to batch")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25465>

src/gallium/drivers/freedreno/freedreno_state.c

@@ -159,8 +159,10 @@ fd_set_constant_buffer(struct pipe_context *pctx, enum pipe_shader_type shader,
       return;
    }
 
-   if (cb->user_buffer && ctx->screen->gen >= 6)
+   if (cb->user_buffer && ctx->screen->gen >= 6) {
       upload_user_buffer(pctx, &so->cb[index]);
+      cb = &so->cb[index];
+   }
 
    so->enabled_mask |= 1 << index;