drm/msm: Attach the IOMMU device during initialization

Everywhere an IOMMU object is created by msm_gpu_create_address_space
the IOMMU device is attached immediately after. Instead of carrying around
the infrastructure to do the attach from the device specific code do it
directly in the msm_iommu_init() function. This gets it out of the way for
more aggressive cleanups that follow.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
[squash in rebase fixups and fix for unused fxn]
Signed-off-by: Rob Clark <robdclark@chromium.org>