drm/msm: use upstream iommu

Downstream kernel IOMMU had a non-standard way of dealing with multiple
devices and multiple ports/contexts.  We don't need that on upstream
kernel, so rip out the crazy.

Note that we have to move the pinning of the ringbuffer to after the
IOMMU is attached.  No idea how that managed to work properly on the
downstream kernel.

For now, I am leaving the IOMMU port name stuff in place, to simplify
things for folks trying to backport latest drm/msm to device kernels.
Once we no longer have to care about pre-DT kernels, we can drop this
and instead backport upstream IOMMU driver.

Signed-off-by: Rob Clark <robdclark@gmail.com>