imx-drm: fix possible hangup when disabling crtcs

- only ever disable the display controller (DC) module after all plane
  IDMAC channels are stopped. This fixes a regression introduced by the
  atomic modeset conversion.
-----BEGIN PGP SIGNATURE-----
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=XhUf
-----END PGP SIGNATURE-----