drm/i915/gmbus: move GPIO enum to gmbus

The GPIO enum is only used in intel_gmbus.c, hide it there.

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/3c386ce08d7d53a45c14c2e7519e4cc78a8161be.1667383630.git.jani.nikula@intel.com