drm/msm/dpu: move SSPP debugfs creation to dpu_kms.c

As SSPP blocks are now visible through dpu_kms->rm.sspp_blocks, move
SSPP debugfs creation from dpu_plane to dpu_kms. We are going to break
the 1:1 correspondence between planes and SSPPs, so it makes no sense
anymore to create SSPP debugfs entries in dpu_plane.c

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>