msm: kgsl: Move the GMU kernel IOMMU device to the GMU platform device

In order to make devlink happy we need to bind an IOMMU device on the
primary GMU platform device.  Since we don't need GMU user yet the
decision is easy.  Move GMU kernel to the platform device instead
of a child.

Change-Id: Ic0dedbad082c57b2aeb6a35573ead8535f3b8ab9
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>