iommu/dart: check return value after calling platform_get_resource()

It will cause null-ptr-deref in resource_size(), if platform_get_resource()
returns NULL, move calling resource_size() after devm_ioremap_resource() that
will check 'res' to avoid null-ptr-deref.
And use devm_platform_get_and_ioremap_resource() to simplify code.

Fixes: 46d1fb07 ("iommu/dart: Add DART iommu driver")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Link: https://lore.kernel.org/r/20220425090826.2532165-1-yangyingliang@huawei.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>