vk_stream_buffer: Fix initializing Vulkan with NVIDIA on Linux

The previous fix only partially solved the issue, as only certain GPUs that needed 9 or less MiB subtracted would work (i.e. GTX 980 Ti, GT 730). This takes from DXVK's example to divide `heap_size` by 2 to determine `allocable_size`. Additionally tested on my Quadro K4200, which previously required setting it to 12 to boot.