I don’t think this is possible. The GPU likely will not function without all memory equipped.
There is no power gating circuit that would turn off individual memory chips. Power savings could only be achieved by setting a lower memory clock (something that is often done by GPU miners to save power)
If you’d physically remove some memory chips, the GPU will likely not work anymore. There is a certain memory bus width that each GPU model requires to be equipped.
nVidia can fuse off parts of the chips it produces, also reducing memory bus width in the process. e.g. the same GPU die can become a Titan XP, a Geforce 1080Ti or a Geforce 1080. They differ in the total amount of memory (chip count) and memory bus width, yet these GPUs are made from the same silicon die.