drm/i915: don't try DP_LINK_BW_5_4 on HSW ULX

Because the docs say ULX doesn't support it on HSW.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>