drm/i915/opregion: early exit from encoder notify if SWSCI isn't there

Newer platforms aren't supposed to have mailbox #2 or SWSCI
support. Bail out early from encoder notify if that is the case,
skipping the out-of-bounds checks and debug messages.

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/82f63313057ed2d96856b3f8d76536826a897bc7.1644489329.git.jani.nikula@intel.com