Your undergrad degree does not affect the selection of B Dent students. It's all based on your GAMSAT score and interview performance... and since the interview is a multiple mini interview combined with medical students, they no longer ask you about your personal history or experiences.
However having said that, doing a science degree will give you a step ahead for the GAMSAT as well as studying the dentistry course which is very intense and they expect you to know a lot...
Basically students without a science background has much more workload and self study time to understand the basic concepts that gets thrown at you. There is extra classes that these students have to attend on top of the 30 ~ 35 hours weeks.
I recommend any degree that has the following subjects,
Chemistry, Physics, Physiology, Anatomy, Immunology, Microbiology, Pathology, Pharmacology because these are the basic life science subjects you'll be learning in Dentistry.