No need for a reboot, echo 9999 >/sys/module/ttm/parameters/pages_limit

You're talking about an allocator policy for when to allow GTT and when not, not the old firmware-level VRAM split thing where whatever size the BIOS sets for VRAM is permanently away from the CPU. The max GTT limit is there to decrease accidental footguns, it's not a technological limitation; at least earlier the default policy was to reserve 1/4 of RAM for non-GPU use, and 1/4*128 GB=32GB is more than enough so you're looking to adjust the policy. It's just an if statement in the kernel, GTT the mechanism doesn't limit it, and deallocating a chunk of memory used by the GPU returns it to the general kernel memory pool, where it can next be used by the CPU again.