drm/i915/display: Abstract C10/C20 pll hw readout

intel_cx0_phy.[ch] should contain the details about C10/C20, not leaking
it to the rest of the driver. Start abstracting this by exporting a
single PLL hw readout that handles the differences between C20 and C10
internally to that compilation unit.

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/20231018222831.4132968-2-lucas.demarchi@intel.com