iommu/mediatek: Move pgtable allocation into domain_alloc

After adding the global list for M4U HW, We get a chance to
move the pagetable allocation into the mtk_iommu_domain_alloc.
Let the domain_alloc do the right thing.

This patch is for fixing this problem[1].
[1]: https://patchwork.codeaurora.org/patch/53987/



Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>