iommu/mediatek: Gather iova in iommu_unmap to achieve tlb sync once

In current iommu_unmap, this code is:

	iommu_iotlb_gather_init(&iotlb_gather);
	ret = __iommu_unmap(domain, iova, size, &iotlb_gather);
	iommu_iotlb_sync(domain, &iotlb_gather);

We could gather the whole iova range in __iommu_unmap, and then do tlb
synchronization in the iommu_iotlb_sync.

This patch implement this, Gather the range in mtk_iommu_unmap.
then iommu_iotlb_sync call tlb synchronization for the gathered iova range.
we don't call iommu_iotlb_gather_add_page since our tlb synchronization
could be regardless of granule size.

In this way, gather->start is impossible ULONG_MAX, remove the checking.

This patch aims to do tlb synchronization *once* in the iommu_unmap.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210107122909.16317-7-yong.wu@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>