x86: ioapic: Avoid redundant lookup of irq_cfg

The caller of ioapic_register_intr() has a pointer to the irq_cfg for
the irq already. Hand it in to avoid a full lookup.

In msi_compose_msg() the pointer to irq_cfg is already available. No
need to look it up again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>