drm/i915/dp_mst: Fix encoder HW state readout for UHBR MST

The encoder HW/SW state verification should use a SW state which stays
unchanged while the encoder/output is active. The intel_dp::is_mst flag
used during state computation to choose between the DP SST/MST modes can
change while the output is active, if the sink gets disconnected or the
MST topology is removed for another reason. A subsequent state
verification using intel_dp::is_mst leads then to a mismatch if the
output is disabled/re-enabled without recomputing its state.

Use the encoder's active MST link count instead, which will be always
non-zero for an active MST output and will be zero for SST.

Fixes: 35d2e4b7 ("drm/i915/ddi: start distinguishing 128b/132b SST and MST at state readout")
Fixes: 40d489fa ("drm/i915/ddi: handle 128b/132b SST in intel_ddi_read_func_ctl()")
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250224093242.1859583-1-imre.deak@intel.com