drm/atomic: Document __drm_planes_state state pointer

drm/atomic: Document __drm_planes_state state pointer

While the old and new state pointers are somewhat self-explanatory, the
state pointer and its relation to the other two really isn't.

Now that we've cleaned up everything and it isn't used in any
modesetting path, we can document what it's still useful for: to free
the right state when we free the global state.

Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-9-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>