drm/radeon: fix lockup when BOs aren't part of the VM on release

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>