iommu: Don't use sme_active() in generic code

Switch to the generic function mem_encrypt_active() because
sme_active() is x86 specific and can't be called from
generic code on other platforms than x86.

Fixes: 2cc13bb4 ("iommu: Disable passthrough mode when SME is active")
Signed-off-by: Joerg Roedel <jroedel@suse.de>