iommu/vt-d: Fix compile warning

Make intel_svm_unbind_mm() a static function.

Fixes: 064a57d7 ("iommu/vt-d: Replace intel SVM APIs with generic SVA APIs")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Acked-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/1590689031-79318-1-git-send-email-jacob.jun.pan@linux.intel.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>