drm/tegra: rgb: Fix the unbound reference count

The of_get_child_by_name() increments the refcount in tegra_dc_rgb_probe,
but the driver does not decrement the refcount during unbind. Fix the
unbound reference count using devm_add_action_or_reset() helper.

Fixes: d8f4a9ed ("drm: Add NVIDIA Tegra20 support")
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20250205112137.36055-1-biju.das.jz@bp.renesas.com