msm: kgsl: disable gpummu pagefault interrupts when iommu is enabled

GPU MMU is used to control access to IOMMU registers on msm8960.
When pagefaults occurs, accessing mmu_pagefault which is NULL
causes kernel panic. Disable gpummu pagefault interrupts, since
pagefaults are handled through iommu fault handler.

CRs-Fixed: 380470
Change-Id: I9717f4af10b9722b0a14ba5995ee2293ae1de09f
Signed-off-by: Anoop Kumar Yerukala <ayeruk@codeaurora.org>