drm/msm/mdp5: Find correct node for creating gem address space

Creating the msm gem address space requires a reference to the dev where
the iommu is located.  The driver currently assumes this is the same as
the platform device, which breaks when the iommu is outside of the
platform device (ie in the parent).  Default to using the platform device,
but check to see if that has an iommu reference, and if not, use the parent
device instead.  This should handle all the various iommu designs for
mdp5 supported systems.

Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
Tested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>