drm/ttm: cleanup ttm_buffer_object_transfer

The function is always called with deleted BOs.

While at it cleanup the indentation as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: xinhui pan <xinhui.pan@amd.com>
Link: https://patchwork.freedesktop.org/patch/352743/