drm/msm/iommu: Fix null pointer dereference in no-IOMMU case

In the case that no IOMMU is available, msm_iommu_new can return NULL
which should be handled. Without we will get a NULL pointer dereference
in the next statement when we try to use the mmu variable.

Fixes: 8cceb77 ("drm/msm/adreno: stall translation on fault for all GPU families")
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/535915/
Link: https://lore.kernel.org/r/20230508-6-4-drm-msm-iommu-fixup-v1-1-a7a21ec94f5b@z3ntu.xyz
Signed-off-by: Rob Clark <robdclark@chromium.org>

Author: z3ntu

drivers/gpu/drm/msm/msm_iommu.c

@@ -410,7 +410,7 @@ struct msm_mmu *msm_iommu_gpu_new(struct device *dev, struct msm_gpu *gpu, unsig
 	struct msm_mmu *mmu;
 
 	mmu = msm_iommu_new(dev, quirks);
-	if (IS_ERR(mmu))
+	if (IS_ERR_OR_NULL(mmu))
 		return mmu;
 
 	iommu = to_msm_iommu(mmu);