drm/i915/opregion: handle SWSCI Mailbox #2 obsoletion

Opregion Mailbox #2 is obsolete for SWSCI usage in opregion v2.x, and
repurposed in opregion v3.x. Warn about obsole mailbox presence in v2.x,
and ignore with an error for v3.x.

v2: Demote drm_warn() to drm_dbg() on opregion v2.x

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/20220210161603.647254-1-jani.nikula@intel.com