drm/xe: Fix NULL check in xe_ggtt_init()

The null check for GT is after calling gt_to_xe, fix it.

Fixes: 3121fed0 ("drm/xe: Cleanup some layering in GGTT")
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240306052002.311196-2-matthew.brost@intel.com