drm/amdgpu: Move compute vm bug logic to amdgpu_vm.c

  In review, Christian would like to keep the logic
  inside amdgpu_vm.c with a cost of slightly slower.
  The loop is still optimized out with this patch.

v2: remove the if statement. Now it is not slower.

Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Christian König <christian.koeng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>