drm/i915/cx0: Sanitize calculating C20 PLL state from tables

drm/i915/cx0: Sanitize calculating C20 PLL state from tables

A follow up change adds a computation for the C20 PLL VDR state, which
is common to both the HDMI algorithmic and DP/HDMI table based method.
To prepare for that streamline the code. The C10 counterpart would
benefit from the same change, leave that for later adding a TODO
comment.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20251117104602.2363671-5-mika.kahola@intel.com