radv: Only use 32 KiB per threadgroup on Stoney.
Causes hangs on some machines.
What works for dEQP-VK.tessellation.shader_input_output.barrier:
- running num_patches = 6 (which limits LDS to 32 KiB)
- running num_patches = 8, and artificially cutting LDS size at 32 KiB.