drm/i915: Fix the i915_dsc_fec_support debugfs file for DP MST connectors

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