spapr: Pass sPAPR machine state down to spapr_pci_switch_vga()

This allows to drop a user of qdev_get_machine().

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20201209170052.1431440-4-groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>