drm/i915/mst: Check intel_dp_joiner_needs_dsc()

drm/i915/mst: Check intel_dp_joiner_needs_dsc()

intel_dp_mst_compute_config() is missing the "does the
joiner need DSC?" check despite claiming to have a lot
of other joiner/dsc stuff in there (albeit disabled).
Replicate the logic from the SST side.

TODO: refactor all this duplicated code!

Tested-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404213441.17637-10-ville.syrjala@linux.intel.com