drm/msm/dp: cleanup debugfs handling

drm/msm/dp: cleanup debugfs handling

Currently there are two subdirs for DP debugfs files, e.g. DP-1, created
by the drm core for the connector, and the msm_dp-DP-1, created by the
DP driver itself. Merge those two, so that there are no extraneous
connector-related subdirs.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/563523/
Link: https://lore.kernel.org/r/20231019104419.1032329-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>