If we unmap BOs before releasing them them the intervall tree locks
up because we try to remove an entry not inside the tree.

Based on a patch from Michel Dänzer.
Signed-off-by: Christian König <christian.koenig@amd.com>
CC: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>