drm/i915: Kill off the BXT buf_trans default_index

default_index contained in the BXT buf_trans tables is actually useless.
For DP we should always have a valid level selected (otherwise the link
training logic would be buggy), and for HDMI we can just do what the
other platforms do and pick the correct entry in intel_ddi_hdmi_level().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171016145705.11780-8-ville.syrjala@linux.intel.com


Reviewed-by: James Ausmus <james.ausmus@intel.com>