drm/xe: Encapsulate drm_mm_node inside xe_ggtt_node

drm/xe: Encapsulate drm_mm_node inside xe_ggtt_node

The xe_ggtt component uses drm_mm to manage the GGTT.
The drm_mm_node is just a node inside drm_mm, but in Xe we use that
only in the GGTT context. So, this patch encapsulates the drm_mm_node
into a xe_ggtt's new struct.

This is the first step towards limiting all the drm_mm access
through xe_ggtt. The ultimate goal is to have a better control of
the node insertion and removal, so the removal can be delegated
to a delayed workqueue.

v2: Fix includes and typos (Michal and Brost)

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240821193842.352557-5-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>