drm/msm: potential error pointer dereference in init()

The msm_iommu_new() returns error pointers on failure so check for that
to avoid an Oops.

Fixes: ccac7ce3

 ("drm/msm: Refactor address space initialization")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20211004103806.GD25015@kili


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>