drm/msm: Consider case of no smmu_client in SMMU

Not all msm_smmu device have client, Only SMMU device with smmu_client
could set fault handler and initialize drm-mm allocator.

Change-Id: Ic6115e01e56040c87eb4e70c875b4fe6356e10f2
Signed-off-by: Kasin Li <donglil@codeaurora.org>