drm/i915/display: Store hw clock for C20

We can calculate the hw port clock during the hw readout
and store it as pll_state->clock for C20 state verification.
In order to do that we need to move intel_c20pll_calc_port_clock()
function.

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240102115741.118525-3-mika.kahola@intel.com