drm/i915/display: Abstract C10/C20 pll calculation

As done with the hw readout, properly abstract the C10/C20 phy details
inside intel_cx0_phy.c.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231018222831.4132968-3-lucas.demarchi@intel.com