drm/panthor: Support partial unmaps of huge pages

Commit 33729a5fc0ca ("iommu/io-pgtable-arm: Remove split on unmap
behavior") did away with the treatment of partial unmaps of huge IOPTEs.

In the case of Panthor, that means an attempt to run a VM_BIND unmap
operation on a memory region whose start address and size aren't 2MiB
aligned, in the event it intersects with a huge page, would lead to ARM
IOMMU management code to fail and a warning being raised.

Presently, and for lack of a better alternative, it's best to have
Panthor handle partial unmaps at the driver level, by unmapping entire
huge pages and remapping the difference between them and the requested
unmap region.

This could change in the future when the VM_BIND uAPI is expanded to
enforce huge page alignment and map/unmap operational constraints that
render this code unnecessary.

When a partial unmap for a huge PTE is attempted, we also need to expand
the locked region to encompass whole huge pages.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251217213252.677020-2-adrian.larumbe@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

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