msm: kgsl: Dont attach pagetable for unused iommu contextbank

IOMMU driver assumes context bank 2 to be secured, if for some
reason we are not able to create secure pagetable we will try
to attach unsecured pagetable to secure context bank, this
causes a device crash. Fix it by not attaching a pagetable to
secure context bank if secure pagetable does not exist.

Change-Id: I5bd150e5f53ea7e33d230af5dc4caf997625193f
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>