drm/bridge: ti-sn65dsi86: make use of debugfs_init callback

Do not create a custom directory in debugfs-root, but use the
debugfs_init callback to create a custom directory at the given place
for the bridge. The new directory layout looks like this on a Renesas
GrayHawk-Single with a R-Car V4M SoC:

	/sys/kernel/debug/dri/feb00000.display/DP-1/1-002c

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250315201651.7339-2-wsa+renesas@sang-engineering.com