iommu/amd: Introduce first_pte_l7() helper

Given an arbitrary pte that is part of a large mapping, this function
returns the first pte of the series (and optionally the mapped size and
number of PTEs)
It will be re-used in a subsequent patch to replace an existing L7
mapping.

Fixes: 6d568ef9 ("iommu/amd: Allow downgrading page-sizes in alloc_pte()")
Signed-off-by: Andrei Dulea <adulea@amazon.de>