drm/amdgpu/ttm: remove man->bdev references.

Just store the device in the private so the link
can be removed from the manager

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-53-airlied@gmail.com