drm/i915/ddi: handle 128b/132b SST in intel_ddi_read_func_ctl()

We'll only ever get here in MST mode from MST stream encoders; the
primary encoder's ->get_config() won't be called when we've detected
it's MST.

v2: Read mst_master_transcoder in 128b/132b SST path (Imre)

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/436854c0bb6ab5c14c3d3837694ea60ac2fbaba2.1735912293.git.jani.nikula@intel.com