drm/msm/dpu: drop enum dpu_mdp and MDP_TOP value

Since there is always just a single MDP_TOP instance, drop the enum
dpu_mdp and corresponding index value.

Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Tested-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/545357/
Link: https://lore.kernel.org/r/20230704022136.130522-6-dmitry.baryshkov@linaro.org