msm; kgsl: Correct secure pagetable pointer check

Secure pagetable pointer in case of secure pagetable creation
failure is set to ERR_PTR. Correct the secure pagetable pointer
check in kgsl_iommu_map_secure_global to avoid invalid pointer
access when someone tries to map a secure global buffer.

Change-Id: Ic938072e38c410a31158b8ab83f2678b2a64f276
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>