drm/msm: Attach the MMUs as soon as they are allocated

Currently the normal and secure MMUs are allocated when the
address space is created in msm_gpu_init() but not attached
until the end of adreno_gpu_init(). Since we can't map buffer
objects in the IOMMU without attaching it first this restricts
when we can allocate buffer objects in the sequence.

For arm-smmu based targets there isn't any reason why we can't
immediately attach the MMU after creating the address space -
this makes the whole system immediately available to map memory
and will facilitate moving around global allocations.

Change-Id: Ic0dedbad161396e9d095f3f3d1e4fca2d240a084
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>