drm/i915/gt: Add a routine to iterate over the pagetables of a GTT

In the next patch, we will want to look at the dma addresses of
individual page tables, so add a routine to iterate over them.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210617063018.92802-6-thomas.hellstrom@linux.intel.com