drm/i915/dp: use known correct array size in rate_to_index

I can't think of a real world bug this could cause now, but this will be
required in follow-up work. While at it, change the parameter order to
be slightly more sensible.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/ff5b08f45a72c2247f5326b080027e2f5d8cc4ee.1490712890.git.jani.nikula@intel.com