drm/i915/mst: Limit MST+DSC to TGL+

The MST code currently assumes that glk+ already supports MST+DSC,
which is incorrect. We need to check for TGL+ actually. ICL does
support SST+DSC, but supposedly it can't do MST+FEC which will
also rule out MST+DSC.

Note that a straight TGL+ check doesn't work here because DSC
support can get fused out, so we do need to also check 'has_dsc'.

Cc: stable@vger.kernel.org
Fixes: d51f25eb

 ("drm/i915: Add DSC support to MST path")
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/20240402135148.23011-6-ville.syrjala@linux.intel.com