The problem is generally that on budget machines like that, the OEM, in this case Dell, doesn't really want users messing around with configuration settings so they come with very limited BIOS options. If you don't see a setting in the BIOS for the graphics memory configuration, then it's probably because it's not there. I don't see anything that leads me to believe it's an option either. The Intel HD graphics should use whatever they need, up to the maximum amount allocated by design.