drm/msm/dpu: Avoid calling _dpu_kms_mmu_destroy() on init failure

Fix the error paths in _dpu_kms_mmu_init() to properly
clean up the iommu domain and not call _dpu_kms_mmu_destroy() when
things are only partially setup.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617200405.131843-2-sean@poorly.run