drm/imx: fix DP CSC handling

- Fix the DP color space conversion matrix setup to avoid bugs where
  disabling the overlay plane while both primary and overlay plane are
  routed via the CSC unit would not reconfigure the CSC routing
  properly, leaving the display in a nonworking state, or the CSC
  setting from a previously set mode would be left behind, causing
  wrong colors when reenabling the display in certain configurations.
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