drm/xe/userptr: Stub notifier_lock helpers when DRM_GPUSVM=n

drm/xe/userptr: Stub notifier_lock helpers when DRM_GPUSVM=n

When CONFIG_DRM_GPUSVM=n (e.g. um-allyesconfig), the only caller of
xe_pt_svm_userptr_notifier_lock() is compiled out, triggering:

  drivers/gpu/drm/xe/xe_pt.c:1418:13: warning:
    'xe_pt_svm_userptr_notifier_lock' defined but not used
    [-Wunused-function]

The helpers cannot simply be removed in this case: the matching
xe_pt_svm_userptr_notifier_unlock() is also referenced from
xe_pt_update_ops_run(), which lives outside any DRM_GPUSVM ifdef and is
gated only at runtime by pt_update_ops->needs_svm_lock. The symbol must
exist in all builds.

Provide empty static inline stubs for !DRM_GPUSVM, matching the pattern
used by xe_svm_notifier_lock()/_unlock() in xe_svm.h.

Fixes: 80ccbd97ffee ("drm/xe/userptr: Hold notifier_lock for write on inject test path")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202606302210.QqcLbOEN-lkp@intel.com/
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260630192221.2998168-1-shuicheng.lin@intel.com
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>