drm/i915: use octal permissions in display debugfs

Octal permissions are preferred over the symbolics ones.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231205134143.2427661-3-jani.nikula@intel.com