drm/i915: Call primary encoder's .get_config() from MST .get_config()

Stop assuming intel_ddi_get_config() is all we need from the primary
encoder, and instead call it via the .get_config() vfunc. This
will allow customized .get_config() for the primary, which I plan
to use to handle the differences in the clock readout between various
platforms.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210224144214.24803-2-ville.syrjala@linux.intel.comReviewed-by: Mika Kahola <mika.kahola@intel.com>

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -591,7 +591,7 @@ static void intel_dp_mst_enc_get_config(struct intel_encoder *encoder,
 	struct intel_dp_mst_encoder *intel_mst = enc_to_mst(encoder);
 	struct intel_digital_port *dig_port = intel_mst->primary;
 
-	intel_ddi_get_config(&dig_port->base, pipe_config);
+	dig_port->base.get_config(&dig_port->base, pipe_config);
 }
 
 static bool intel_dp_mst_initial_fastset_check(struct intel_encoder *encoder,