drm/amdgpu: stop evicting encrypted BOs to swap

Swapping out encrypted BOs doesn't work because they can't change
their physical location without going through a bounce copy.

As a workaround disable evicting encrypted BOs to the system
domain for now.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>