drm/amdgpu: Move dmabuf attach/detach to backend_(un)bind

The dmabuf attachment should be updated by moving the SG BO to DOMAIN_CPU
and back to DOMAIN_GTT. This does not necessarily invoke the
populate/unpopulate callbacks. Do this in backend_bind/unbind instead.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>