iommu/ipmmu-vmsa: Rereserving a free context before setting up a pagetable

Reserving a free context is both quicker and more likely to fail
(due to limited hardware resources) than setting up a pagetable.
What is more the pagetable init/cleanup code could require
the context to be set up.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
CC: Robin Murphy <robin.murphy@arm.com>
CC: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
CC: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>