drm/msm: Refactor address space initialization

Refactor how address space initialization works. Instead of having the
address space function create the MMU object (and thus require separate but
equal functions for gpummu and iommu) use a single function and pass the
MMU struct in. Make the generic code cleaner by using target specific
functions to create the address space so a2xx can do its own thing in its
own space.  For all the other targets use a generic helper to initialize
IOMMU but leave the door open for newer targets to use customization
if they need it.

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