Trying to evict things from the current working set doesn't work that
well anymore because of per VM BOs.

Rely on reserving VRAM for page tables to avoid contention.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>