drm/i915/bios: intel_bios_hdmi_boost_level()

Don't access i915->vbt.ddi_port_info[] directly.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6d61a5bc60c995d2ee812ef61d3c5c93b61453e7.1579270868.git.jani.nikula@intel.com