x86: don't call pxm_to_node again

also make bus_numa work even if ACPI_NUMA is not defined.

don't call pxm_to_node again, and use node directly.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>