drm/msm: Limit command submission when no IOMMU

Running the GPU without an IOMMU is not really a supported (or sane)
configuration.  Yet it can be useful during SoC bringup (ie. if the
iommu driver doesn't work yet).

Lets limit it to users who already have /dev/mem access, to avoid the
chance that a user accidentially configures kernel without IOMMU
support.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20220502172908.3569799-1-robdclark@gmail.com


Signed-off-by: Rob Clark <robdclark@chromium.org>