drm/amdgpu: Add new placement for preemptible SG BOs

SG BOs such as dmabuf imports and userptr BOs do not consume system
resources directly. Instead they point to resources owned elsewhere.
They typically get evicted by DMABuf move notifiers of MMU notifiers.
If those notifiers don't need to wait for hardware fences (i.e. the SG
BOs are used in a preemptible context), then we don't need to limit
them to the GTT size and we don't need TTM to evict them.

Create a new placement for such preemptible SG BOs that does not impose
artificial size limits and TTM evictions.

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