drm/msm/dpu: constify struct dpu_mdss_cfg

drm/msm/dpu: constify struct dpu_mdss_cfg

Mark struct dpu_mdss_cfg instance as a const pointer. This is mostly a
preparation for the next patch.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/488164/
Link: https://lore.kernel.org/r/20220602202447.1755115-7-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>