drm/i915/opregion: debug log about Mailbox #2 for backlight

Start debug logging about the presence of the new Mailbox #2 for
backlight. Actual support is to be added later.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/42fd9cd777c5cc9a8d48db9dd8306924c735918e.1644489329.git.jani.nikula@intel.com