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

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: 8cceb773f565 ("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>