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.

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>