drm/ttm: Account for kernel allocations in kernel zone only

Don't account for them in other zones such as dma32. The kernel page
allocator has its own heuristics to avoid exhausting special zones
for regular kernel allocations.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>