drm/msm/dpu: drop dpu_plane_pipe function

There no more need for the dpu_plane_pipe() function, crtc code can
access pstate->pipe_hw.idx directly.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Tested-by: Abhinav Kumar <quic_abhinavk@quicinc.com> # sc7280
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/527320/
Link: https://lore.kernel.org/r/20230316161653.4106395-7-dmitry.baryshkov@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>