drm/msm/mdp4: move iommu_domain_alloc() call close to its usage

Move iommu_domain_alloc() in front of adress space/IOMMU initialization.
This allows us to drop it from struct mdp4_cfg_platform which
remained from the pre-DT days.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/485049/
Link: https://lore.kernel.org/r/20220505135008.1351533-2-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>