Split tc_pxl_pll_en() into tc_pxl_pll_calc() which does only Pixel PLL
parameter calculation and tc_pxl_pll_en() which calls tc_pxl_pll_calc()
and then configures the Pixel PLL register.

This is a preparatory patch for further rework, where tc_pxl_pll_calc()
will also be used to find out the exact clock frequency generated by the
Pixel PLL. This frequency will be used as adjusted_mode clock frequency
and passed down the display pipeline to obtain exactly this frequency
on input into this bridge.

The precise input frequency that matches the Pixel PLL frequency is
important for this bridge, as if the frequencies do not match, the
bridge does suffer VFIFO overruns or underruns.
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240625120334.145320-1-marex@denx.de