winsys/amdgpu: optimize slab allocation for 2 MB amdgpu page tables

winsys/amdgpu: optimize slab allocation for 2 MB amdgpu page tables

- the slab buffer size increased from 128 KB to 2 MB (PTE fragment size)
- the max suballocated buffer size increased from 64 KB to 256 KB,
  this increases memory usage because it wastes memory
- the number of suballocators increased from 1 to 3 and they are layered
  on top of each other to minimize unused space in slabs

The final increase in memory usage is:
  DeusEx:MD:  1.8%
  DOTA 2:     1.75%
  DiRT Rally: 0.2%

The kernel driver will also receive fewer buffers.