DSC is not supported on DP MST streams so just don't add this entry for
MST connectors.

This also fixes an OOPS, caused by the encoder->digport cast, which is
not valid for MST encoders.

v2:
- Check encoder, which is unset for an MST connector, before it gets
  enabled.
v3:
- Just don't add this debugfs file for MST connectors. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200609184140.4937-1-imre.deak@intel.com