drm/bridge: ti-sn65dsi86: Clean debugfs code

Let's cleanup the debugfs code to:
- Check for errors.
- Use devm to manage freeing, which also means we don't need to store
  a pointer in our structure.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210423095743.v5.5.I5fe072753290c6a77eda736ebd5778e17b7cb0fb@changeid