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