I dont know if there is a ferry to Luanda from Cabinda, but for sure there is one to Soyo.
This ferry takes about six hours and as far as I remember (we travelled in 02/2005 coming from Soyo) it goes daily.
You wont be able to put a car on it, motorcycles only.
Soyo to Luanda the road is not easy in the wet season, but also no drama with an enduro.
If I remember correct the price was around 80 usd for two bike and two riders. He asked first for 120 usd but after talking to head of the company, 80 usd was ok, maybe because I had a broken leg in a plaster.