drm/i915/cnl: Fix DDI hdmi level selection.

Let's get a proper HDMI DDI entry level for vswing programming
sequences on CNL.

Spec doesn't specify any default for HDMI tables,
so let's pick the last entry as the default for now.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170829232230.23051-7-rodrigo.vivi@intel.com