drm/i915/bios: add intel_bios_hdmi_level_shift()

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/da8ca144020fe165af33992661568d0586a2fdeb.1579270868.git.jani.nikula@intel.com