drm/msm: Attach the GPU MMU when it is created

Currently the GPU MMU is attached in the adreno_gpu code but as
more and more of the GPU initialization moves to the generic
GPU path we have a need to map and use GPU memory earlier and
earlier.  There isn't any reason to defer attaching the MMU
until later so attach it right after the address space is
created so it can be used immediately.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>