drm/amdgpu: Use preemptible placement for KFD

KFD userptr BOs and SG BOs used for DMA mappings can be preempted with
CWSR. Therefore we can use preemptible placement and avoid unwanted
evictions due to GTT accounting.

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