drm/i915/gmbus: combine gmbus pin lookups to one function

Combine the platform specific if ladders for array lookup and size
checks into one. This is cleaner and avoids duplication, but hopefully
also helps any static analyzers that seem to have trouble with the
bounds checks.

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/20220303181931.1661767-1-jani.nikula@intel.com