drm/amd/display: fix wrong statement in mst hpd debugfs

[why]
Previous statement would always evaluate to true
making it meaningless
[how]
Just check if a connector is MST by checking if its port exists.

Fixes: 41efcd3 ("drm/amd/display: Add MST capability to trigger_hotplug interface")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Wayne Lin <waynelin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Mikita Lipski

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_debugfs.c

@@ -3012,7 +3012,7 @@ static int trigger_hpd_mst_set(void *data, u64 val)
 			if (!aconnector->dc_link)
 				continue;
 
-			if (!(aconnector->port && &aconnector->mst_port->mst_mgr))
+			if (!aconnector->mst_port)
 				continue;
 
 			link = aconnector->dc_link;