drm/i915/ddi: clarify intel_ddi_connector_get_hw_state() for DP MST

drm/i915/ddi: clarify intel_ddi_connector_get_hw_state() for DP MST

encoder->get_hw_state() returns false for DP MST, and currently always
interprets 128b/132b as MST. Therefore the DDI MST mode checks in
intel_ddi_connector_get_hw_state() are redundant.

Prepare for future, and handle 128b/132b SST and warn on 8b/10b MST.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241125120959.2366419-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>