Both Ice Lake and Elkhart Lake (gen 11) support MST on all external
connections except DDI A. Tiger Lake (gen 12) supports on all external
connections.

Move the check to happen inside intel_dp_mst_encoder_init() and add
specific platform checks.

v2: Replace != with == checks for ports on gen < 11 (Ville)
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015164029.18431-3-lucas.demarchi@intel.com