drm/i915/dp_mst: Disable DSC on ICL MST outputs

Enabling DSC on ICL MST outputs is broken leading to FIFO pipe /
transcoder underruns and blank screen. On TGL+ platforms MST/DSC works -
after fixing the known issues in [1] - however to make this work on ICL
requires more work.

So far DSC on MST probably didn't get enabled for users - due to an issue
fixed by [2] - but after fixing that, DSC could get enabled, leading to a
blank screen in ICL/MST configurations which do work atm. To prevent
this disable MST/DSC on ICL for now.

[1] https://lore.kernel.org/all/20231030155843.2251023-1-imre.deak@intel.com
[2] https://lore.kernel.org/all/20231030155843.2251023-31-imre.deak@intel.com

v2 (Ville):
- Use DISPLAY_VER >= 12 instead of > 11.
- Explain the ICL DSC issue in code comment.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231102194434.2634786-1-imre.deak@intel.com