drm/amdgpu: Fix possible null pointer dereference

mem = bo->tbo.resource may be NULL in amdgpu_vm_bo_update.

Fixes: 18025378

 ("drm/ttm: stop allocating dummy resources during BO creation")
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org