drm/i915: Remove the fragile array index -> link rate mapping

Rather than try to maintain some magic relationship between the link
rates and the index into the wrpll params array let's just store
the link rate in the array itself. Much less fragile.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190207173230.22368-13-ville.syrjala@linux.intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>