cxl: Destroy afu->contexts_idr on release of an afu

Destroy afu->contexts_idr on release of an afu, reclaiming the allocated
memory.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>