clk: imx: Let IMX8MN_CLK_DISP_PIXEL set parent rate

By default the display pixel clock needs to be evenly divide
down from the video_pll_out clock which rules out a significant
number of resolution and refresh rates.

The current clock tree looks something like:

 video_pll                594000000
  video_pll_bypass        594000000
   video_pll_out          594000000
    disp_pixel            148500000
     disp_pixel_clk       148500000

Now that composite-8m supports determine_rate, we can allow
disp_pixel to set the parent rate which then switches
every clock in the chain to a new frequency when disp_pixel
cannot evenly divide from video_pll_out.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20230323230127.120883-5-aford173@gmail.com