drm/xe: Remove xe_ggtt_node_allocated
With the intermediate state gone, no longer useful. Just check against
NULL where needed.
After looking carefully, the check for allocated in xe_fb_pin.c is
unneeded. vma->node is never NULL. The check is specifically only
to check if vma->node == the bo's root tile ggtt_obj.
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patch.msgid.link/20260206112108.1453809-12-dev@lankhorst.se
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>