drm/i915/gmbus: reduce gmbus pin lookups in gmbus setup

Avoid separate pin lookups for validity and name.

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/20220303181931.1661767-2-jani.nikula@intel.com