drm/amdgpu: Handle NULL bo->tbo.resource (again) in amdgpu_vm_bo_update

Third time's the charm, I hope?

Fixes: d3116756 ("drm/ttm: rename bo->mem and make it a pointer")
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3837


Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 695c2c74)
Cc: stable@vger.kernel.org