drm/display/dp_mst: Fix confusing docs for drm_dp_atomic_release_time_slots()

For some reason we mention returning 0 if "slots have been added back to
drm_dp_mst_topology_state->avail_slots". This is totally misleading,
avail_slots is simply for figuring out the total number of slots available
in total on the topology and has no relation to the current payload
allocations.

So, let's get rid of that comment.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Fangzhi Zuo <Jerry.Zuo@amd.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <sean@poorly.run>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220817193847.557945-6-lyude@redhat.com