drm/modes: Fix drm_mode_copy() docs

There is no object id in drm_display_mode anymore.
Remove stale comments to the contrary.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209091928.14766-2-ville.syrjala@linux.intel.com
Acked-by: Maxime Ripard <maxime@cerno.tech>