drm/msm: implement smmu detach and guard freeing of client dev

Provide a basic implementation of msm_smmu_detach. Check
that smmu client_dev exists before doing
platform_device_unregister.

Change-Id: Ied1481003d878af452d3b00b950b234c6b7dd698
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>