iommu/amd: Remove the special case from alloc_irq_table()

alloc_irq_table() has a special ioapic argument. If set then it will
pre-allocate / reserve the first 32 indexes. The argument is only once
true and it would make alloc_irq_table() a little simpler if we would
extract the special bits to the caller.
The caller of irq_remapping_alloc() is holding irq_domain_mutex so the
initialization of iommu->irte_ops->set_allocated() should not race
against other user.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>