drm/msm/dpu: Use provided drm_minor to initialize debugfs

Instead of reaching into dev->primary for debugfs_root, use the minor
passed into debugfs_init.

This avoids creating the debug directory under /sys/kernel/debug/ and
instead creates the directory under the correct node in
/sys/kernel/debug/dri/<node>/

Reported-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524173231.5040-1-sean@poorly.run