drm/i915: Require an exact DP link freq match for the DG2 PLL

No idea why the DG2 PLL DP link frequency calculation is allowing
a non-exact match. That makes no sense so get rid of it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220503182242.18797-24-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Matt Roper <matthew.d.roper@intel.com>