drm/gm12u320: Use helpers for shutdown/suspend/resume

Also there's a race in the disconnect implemenation. First shut
down, then unplug, leaves a window where userspace could sneak
in and restart the entire machinery.

With this we can also delete the very un-atomic global pipe_enabled
tracking.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-45-daniel.vetter@ffwll.ch