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

include/drm/drm_dp_mst_helper.h

@@ -589,6 +589,11 @@ struct drm_dp_mst_topology_mgr {
 	 */
 	bool payload_id_table_cleared : 1;
 
+	/**
+	 * @is_waiting_for_dwn_reply: whether we're waiting for a down reply.
+	 */
+	bool is_waiting_for_dwn_reply : 1;
+
 	/**
 	 * @mst_primary: Pointer to the primary/first branch device.
 	 */
@@ -618,11 +623,6 @@ struct drm_dp_mst_topology_mgr {
 	 */
 	struct mutex qlock;
 
-	/**
-	 * @is_waiting_for_dwn_reply: indicate whether is waiting for down reply
-	 */
-	bool is_waiting_for_dwn_reply;
-
 	/**
 	 * @tx_msg_downq: List of pending down replies.
 	 */