drm/msm: Fix IOMMU clean up path in case msm_iommu_new() fails

msm_iommu_new() can fail and this change makes sure that we
detect the failure and free the allocated domain before going
any further.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>