Force multi-select position to be on bottom

Vue-Multiselect *should* calculate the multi-select position automatically https://github.com/shentao/vue-multiselect/blob/master/src/multiselectMixin.js#L696-L715

 , but that fails for some reason. We can force the position in this particular case.

Signed-off-by: Thomas Citharel <tcit@tcit.fr>