msm: kgsl: Avoid a NULL dereference in kgsl_iommu_destroy_pagetable

In kgsl_iommu_destroy_pagetable we attempt to get the physical base of the
pagetable without verifying the validity of the iommu_pt->domain pointer.

Change-Id: Ic0dedbad8f09ed09c1fab49c9d0cf1a73b5b33a5
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>