drm/dp_mst: Don't report ports connected if nothing is attached to them

Reporting a port as connected if nothing is attached to them leads to
any i2c transactions on this port trying to use an uninitialized i2c
adapter, fix this.

Let's account for this case even if branch devices have no good reason
to report a port as plugged with their peer device type set to 'none'.

Fixes: db1a0795 ("drm/dp_mst: Handle SST-only branch device case")
References: https://gitlab.freedesktop.org/drm/intel/-/issues/2987
References: https://gitlab.freedesktop.org/drm/intel/-/issues/1963


Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: <stable@vger.kernel.org> # v5.5+
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reported-by: Thiago Macieira <gitlab@gitlab.freedesktop.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210201120145.350258-1-imre.deak@intel.com