drm/i915/skl, cnl: Split out the WRPLL/LCPLL frequency calculation

Split out the PLL parameter->frequency conversion logic for each type of
PLL for symmetry with their corresponding inverse conversion functions.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226203455.23032-12-imre.deak@intel.com