drm/dp_mst: Convert drm_dp_mst_topology_mgr.is_waiting_for_dwn_reply to bitfield

Small nitpick that I noticed a second ago - we can save some space in
the struct by making this a bitfield and sticking it with the rest of
the bitfields. Also, some small cleanup to the kdocs for this member.

There should be no functional changes in this patch.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Wayne Lin <Wayne.Lin@amd.com>
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200122194846.16025-1-lyude@redhat.com