openpic: fix remaining issues from idr-to-destmask conversion

openpic_update_irq() was checking idr rather than destmask, treating
it as if it were a simple bitmap of cpus.  Changed to use destmask.

IPI delivery was removing bits directly from .idr, without calling
write_IRQreg_idr so that the change could be conveyed to destmask.
Changed to use destmask directly.

Save/restore destmask when serializing, as due to the IPI change it
cannot be reproduced from idr.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
