iommu: Fix IOMMU debugfs fallout

A change made in the final version of IOMMU debugfs support replaced the
public function iommu_debugfs_new_driver_dir() by the public dentry
iommu_debugfs_dir in <linux/iommu.h>, but forgot to update both the
implementation in iommu-debugfs.c, and the patch description.

Fix this by exporting iommu_debugfs_dir, and removing the reference to
and implementation of iommu_debugfs_new_driver_dir().

Fixes: bad614b2 ("iommu: Enable debugfs exposure of IOMMU driver internals")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Gary R Hook <gary.hook@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>