ocxl: Access interrupt trigger page from xive directly

We can access the trigger page through standard APIs so let's use it
and avoid saving it when allocating the interrupt. It will also allow
to simplify allocation in a later patch.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200403153838.29224-3-fbarrat@linux.ibm.com