drm/i915: Fix ibx max vswing/preemph

IBX supports vswing level 3 and pre-emphasis level 3. Don't
limit it to level 2 for those.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200512174145.3186-3-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>