drm/i915/icl: Fix signal_levels

Since when it was introduced we forgot to add
this case so ICL was using a wrong signal_levels
as reference.

Fixes: fb5c8e9d

 ("drm/i915/icl: Implement voltage swing programming sequence for Combo PHY DDI")
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181017215652.26841-1-rodrigo.vivi@intel.com