drm/tegra: Shutdown on driver unbind

Since commit 846c7dfc

 ("drm/atomic: Try to preserve the crtc enabled
state in drm_atomic_remove_fb, v2."), removing the last framebuffer will
no longer disable the corresponding pipeline, which causes the KMS core
to complain about leaked connectors on driver unbind.

Fix this by calling drm_atomic_helper_shutdown() on driver unbind, which
will cause all display pipelines to be shut down and therefore drop the
extra references on the connectors.

Signed-off-by: Thierry Reding <treding@nvidia.com>