iommu/vt-d: Use memunmap to free memremap

memunmap() should be used to free the return of memremap(), not
iounmap().

Fixes: dfddb969 ('iommu/vt-d: Switch from ioremap_cache to memremap')
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>