drm/fsl-dcu: use common clock framework for pixel clock divider

Use the common clock framework to calculate the pixel clock
dividier. The previous implementation rounded down the calculated
factor. Thanks to the CLK_DIVIDER_ROUND_CLOSEST flag using the
common clock framework divider implementation improves the pixel
clock accuracy in some cases. Ontop of that it also allows to see
the actual pixel clock in the sysfs clock summary.

Signed-off-by: Stefan Agner <stefan@agner.ch>