drm/i915/bios: intel_bios_dp_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/fb8645cecadbc4ebeea1c0de94cb3116a769d9bf.1579270868.git.jani.nikula@intel.com