acpi: use indirect call to register gsi in different modes

Rather than using a tree of conditionals, use function pointer
for acpi_register_gsi.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>