drm/amdgpu: use moving fence instead of exclusive for VM updates

Make VM updates depend on the moving fence instead of the exclusive one.

Makes it less likely to actually have a dependency.

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>