drm/dp_mst: Tune down the WARN modesetting a port with full_pbn=0

It's possible to modeset a connector/mst port that has a 0 full_pbn
value: if the sink on the port deasserts its HPD and a branch device
reports this via a CSN with the port's ddps=0 and pdt!=NONE the driver
clears full_pbn, but the corresponding connector can be still
modesetted.

This happened on a DELL U2719D monitor as the branch device and an LG
27UL650-W daisy-chained to it, the LG monitor generating a long HPD
pulse (doing this for some reason always when waking up from some power
saving state).

Tune down the WARN about this scenario to a debug message.

v2: Use the correct atomic debug message level. (Lyude)

References: https://gitlab.freedesktop.org/drm/intel/-/issues/1917
Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210216123448.410545-1-imre.deak@intel.com

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -5105,11 +5105,16 @@ drm_dp_mst_atomic_check_port_bw_limit(struct drm_dp_mst_port *port,
 		if (!found)
 			return 0;
 
-		/* This should never happen, as it means we tried to
-		 * set a mode before querying the full_pbn
+		/*
+		 * This could happen if the sink deasserted its HPD line, but
+		 * the branch device still reports it as attached (PDT != NONE).
 		 */
-		if (WARN_ON(!port->full_pbn))
+		if (!port->full_pbn) {
+			drm_dbg_atomic(port->mgr->dev,
+				       "[MSTB:%p] [MST PORT:%p] no BW available for the port\n",
+				       port->parent, port);
 			return -EINVAL;
+		}
 
 		pbn_used = vcpi->pbn;
 	} else {