clk: imx: imx7d: correct video pll clock tree

There is a test divider and post divider in video PLL,
test divider is placed before post divider, all clocks
that can select parent from video PLL should be from
post divider, NOT from pll_video_main, below are
clock tree dump before and after this patch:

Before:
pll_video_main
   pll_video_main_bypass
      pll_video_main_clk
         lcdif_pixel_src
            lcdif_pixel_cg
               lcdif_pixel_pre_div
                  lcdif_pixel_post_div
                     lcdif_pixel_root_clk
After:
pll_video_main
   pll_video_main_bypass
      pll_video_main_clk
         pll_video_test_div
            pll_video_post_div
               lcdif_pixel_src
                  lcdif_pixel_cg
                     lcdif_pixel_pre_div
                        lcdif_pixel_post_div
                           lcdif_pixel_root_clk
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>