drm/i915/ddi: start distinguishing 128b/132b SST and MST at state readout

drm/i915/ddi: start distinguishing 128b/132b SST and MST at state readout

We'll want to distinguish 128b/132b SST and MST modes at state
readout. There's a catch, though. From the hardware perspective,
128b/132b SST and MST programming are pretty much the same. And we can't
really ask the sink at this point.

If we have more than one transcoder in 128b/132b mode associated with
the port, we can safely assume it's MST. But for MST with only a single
stream enabled, we are pretty much out of luck. Let's fall back to
looking at the software state, i.e. intel_dp->is_mst. It should be fine
for the state checker, but for hardware takeover at probe, we'll have to
trust the GOP has only enabled SST.

TODO: Not sure how this *or* our current code handles 128b/132b enabled
by GOP.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/119a773a0d4d74ad204435e462f8d12cb0ea4128.1735912293.git.jani.nikula@intel.com