drm/i915: Use a fixed N value always

Windows/BIOS always uses fixed N values. Let's match that
behaviour.

Allows us to also get rid of that constant_n quirk stuff.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-17-ville.syrjala@linux.intel.com