drm/msm/dpu: move SSPP debugfs support from plane to SSPP code

We are preparing to change DPU plane implementation. Move SSPP debugfs
code from dpu_plane.c to dpu_hw_sspp.c, where it belongs.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Link: https://lore.kernel.org/r/20211201222633.2476780-9-dmitry.baryshkov@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>