drm/msm: don't return NULL from msm_iommu_new()

drm/msm: don't return NULL from msm_iommu_new()

As we've dropped no-IOMMU support, there is no reason to specially
handle the no-IOMMU case inside msm_iommu_new(). Make it return -ENODEV
if there is no IOMMU (instead of returning NULL) and simplify all
calling sites accordingly.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/672561/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>