drm/i915/bios: add BDB block comments before definitions

Use the comments verbatim from the spec to help find the right block. No
functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/52c32be96bd605d7a9f94accbd4dbe7718849f93.1559308269.git.jani.nikula@intel.com