msm: kgsl: Return -ENOEM on error from kgsl_iommu_svm_find_region

A undefined value is returned from kgsl_iommu_svm_find_region() when
a region isn't found which causes problems for the SVM engine
especially when the undefined value hapens to be 0.

Explicitly set the return value to -ENOMEM.

Change-Id: Ic0dedbadda1c541d937ebb0176de3d7bad589bfd
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>