MLK-20707 gpu: imx: dcss: Change the pixel clock reference at runtime

MLK-20707 gpu: imx: dcss: Change the pixel clock reference at runtime

The clock referenece for the video pll 2 needs to be changed at runtime.
The HDMI/DP PHY reference clock is not available during module initialization
so the reference clock should be changed when the mode is set.

Signed-off-by: Oliver Brown <oliver.brown@nxp.com>