iommu/mediatek: Fix 2 HW sharing pgtable issue

In the commit 4f956c97 ("iommu/mediatek: Move domain_finalise into
attach_device"), I overlooked the sharing pgtable case.
After that commit, the "data" in the mtk_iommu_domain_finalise always is
the data of the current IOMMU HW. Fix this for the sharing pgtable case.

Only affect mt2712 which is the only SoC that share pgtable currently.

Fixes: 4f956c97

 ("iommu/mediatek: Move domain_finalise into attach_device")
Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20220503071427.2285-5-yong.wu@mediatek.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>