drm/amdgpu: Update BO eviction priorities

Make SVM BOs more likely to get evicted than other BOs. These BOs
opportunistically use available VRAM, but can fall back relatively
seamlessly to system memory. It also avoids SVM migrations evicting
other, more important BOs as they will evict other SVM allocations
first.

Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Acked-by: Mukul Joshi <mukul.joshi@amd.com>
Tested-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>