drm/i915/tc: Fix TC port link ref init for DP MST during HW readout

An enabled TC MST port holds one TC port link reference, regardless of
the number of enabled streams on it, but the TC port HW readout takes
one reference for each active MST stream.

Fix the HW readout, taking only one reference for MST ports.

This didn't cause an actual problem, since the encoder HW readout doesn't
yet support reading out the MST HW state.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316131724.359612-3-imre.deak@intel.com