drm/msm/dpu: drop dpu_mdss_cfg::mdp_count field

There is always a single MDP TOP block. Drop the mdp_count field and
stop declaring dpu_mdp_cfg instances as arrays.

Tested-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/545355/
Link: https://lore.kernel.org/r/20230704022136.130522-5-dmitry.baryshkov@linaro.org