drm/i915/dp: localize link rate index variable more

Localize link_rate_index to the if block, and rename to just index to
reduce indent.

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/d348d990c96705427b93c1cac8c3e4447d06eebf.1491485983.git.jani.nikula@intel.com