We allocate the collection mapping on device creation, but somehow
free it on the irqdomain free path, which is pretty inconsistent
and has led to bugs in the past.

Move it to the point where we teardown the device, making the
alloc/free symetric.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20191108165805.3071-2-maz@kernel.org