drm/xe: Rewrite GGTT VF initialization

drm/xe: Rewrite GGTT VF initialization

The previous code was using a complicated system with 2 balloons to
set GGTT size and adjust GGTT offset. While it works, it's overly
complicated.

A better approach is to set the offset and size when initializing GGTT,
this removes the need for adding balloons. The resize function only
needs readjust ggtt->start to have GGTT at the new offset.

This removes the need to manipulate the internals of xe_ggtt outside
of xe_ggtt, and cleans up a lot of now unneeded code.

Co-developed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patch.msgid.link/20260206112108.1453809-9-dev@lankhorst.se
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>