iommu/ipmmu-vmsa: Break out domain allocation code

Break out the domain allocation code into a separate function.

This is preparation for future code sharing.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Joerg Roedel <jroedel@suse.de>