cxl: Fix NULL pointer dereference on kernel contexts with no AFU interrupts

If a kernel context is initialised and does not have any AFU interrupts
allocated it will cause a NULL pointer dereference when the context is
detached since the irq_names list will not have been initialised.

Move the initialisation of the irq_names list into the cxl_context_init
routine so that it will be valid for the entire lifetime of the context
and will not cause a NULL pointer dereference.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>