drm/i915: Fix ivb cpu edp vswing

According to the DP spec supporting vswing 1 + preemph 2 is
mandatory. We don't have the hw settings for that though. In
order to pretend to follow the DP spec let's just select
vswing 0 + preemph 2 in this case (the DP spec says to use
the requested preemph in preference to the vswing when the
requested values aren't supported).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200512174145.3186-4-ville.syrjala@linux.intel.com


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