x86: apic: Move probe_nr_irqs_gsi() into ioapic_init_mappings()

probe_br_irqs_gsi() is called right after ioapic_init_mappings() and
there are no other users. Move it into ioapic_init_mappings() so the
declaration can disappear and the function can become static.

Rename ioapic_init_mappings() to ioapic_and_gsi_init() to reflect that
change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <1287510389-8388-2-git-send-email-dirk.brandewie@gmail.com>
Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>