iommu/vt-d: Fix NULL pointer reference in intel_svm_bind_mm()

Intel IOMMU could be turned off with intel_iommu=off. If Intel
IOMMU is off,  the intel_iommu struct will not be initialized.
When device drivers call intel_svm_bind_mm(), the NULL pointer
reference will happen there.

Add dmar_disabled check to avoid NULL pointer reference.

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reported-by: Dave Jiang <dave.jiang@intel.com>
Fixes: 2f26e0a9 ("iommu/vt-d: Add basic SVM PASID support")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>