drm/xe: Add debugfs for dumping GGTT mappings

Adding a debugfs dump of GGTT was useful for some debugging I did,
and easy to add. Might be useful for others too.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>