drm/i915: Fix cpt/ppt max pre-emphasis

drm/i915: Fix cpt/ppt max pre-emphasis

cpt/ppt support pre-emphasis level 3. Let's actually declare
support for it, instead of clamping things to level 2.

Also tweak the if-ladder in intel_dp_voltage_max() to match
intel_dp_pre_emphasis_max() to make it easier to compare them.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200512174145.3186-2-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>