x86: use dstapic in mp_config_acpi_legacy_irqs

so we don't get the same value multiple times.

also make mp_config_acpi_legacy_irqs more readable by moving assignments
together.

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