drm/i915/dp: Use connector DSC DPCD in i915_dsc_fec_support_show()

Use the connector's DSC DPCD capabilities in i915_dsc_fec_support_show()
instead of the version stored in the encoder. Atm the two are identical,
but a follow-up patch will store the (MST) connector specific version
in the connector.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231006133727.1822579-6-imre.deak@intel.com