drm/ttm: rename manager variable to make sure wrapper is used.

Other users of this should notice this change and switch to wrapper.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-44-airlied@gmail.com