drm/msm/dpu: Remove _dpu_debugfs_init

Fold it into dpu_debugfs_init.

Cc: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524173231.5040-2-sean@poorly.run