drm/ttm: Fix ttm_bo_move_memcpy

All error paths will want to keep the mm node, so handle this at the
function exit. This fixes an ioremap failure error path.
Also add some comments to make the function a bit easier to understand.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Cc: stable@vger.kernel.org