The msm_gem_get_vaddr() returns an ERR_PTR() on failure, and a null
is catastrophic here, so we should use IS_ERR_OR_NULL() to check
the return value.

Fixes: 6a8bd08d ("drm/msm: add sudo flag to submit ioctl")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/547712/Signed-off-by: Rob Clark <robdclark@chromium.org>