drm/i915/ddi: 128b/132b SST also needs DP_TP_CTL_MODE_MST

It's not very clearly specified, and the hardware bit is ill-named, but
128b/132b SST also needs the MST mode set in the DP_TP_CTL register.

This is preparation for enabling 128b/132b SST. This path is not
reachable yet.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b29fbba8c979a8bab2bf03088610fe408faaf704.1735912293.git.jani.nikula@intel.com