drm/xe/vf: Use only assigned GGTT region

Each VF is assigned a limited range of the GGTT address space.
To ensure that the VF driver does not use GGTT allocations outside
of the assigned region, explicitly reserve GGTT space below and
above this region when initializing GGTT.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240527112015.1020-1-michal.wajdeczko@intel.com