drm/msm/mdp5: move iommu_domain_alloc() call close to its usage

drm/msm/mdp5: 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 final bits of struct mdp5_cfg_platform which
remained from the pre-DT days.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/489695/
Link: https://lore.kernel.org/r/20220616081106.350262-3-dmitry.baryshkov@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>