drm/xe: Start using ggtt->start in preparation of balloon removal

drm/xe: Start using ggtt->start in preparation of balloon removal

Instead of having ggtt->size point to the end of ggtt, have ggtt->size
be the actual size of the GGTT, and introduce ggtt->start to point to
the beginning of GGTT.

This will allow a massive cleanup of GGTT in case of SRIOV-VF.

Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
Link: https://patch.msgid.link/20260108101014.579906-10-dev@lankhorst.se