drm/i915/opregion: abstract the check for valid swsci function

Add a reusable function for checking the SWSCI function.

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/778f8716ec44adad3cf95a7bb327a7c8e981291d.1644489329.git.jani.nikula@intel.com