drm/i915/dg2: Update to bigjoiner path

In verify_mpllb_state() encoder is retrieved from best_encoder
of connector_state. As there will be only one connector_state
for bigjoiner and checking encoder may not be needed for
bigjoiner-slave. This code path related to mpll is done on dg2
and need this fix to avoid null pointer dereference issue.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723174239.1551352-30-matthew.d.roper@intel.com

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

@@ -9198,6 +9198,9 @@ verify_mpllb_state(struct intel_atomic_state *state,
 	if (!new_crtc_state->hw.active)
 		return;
 
+	if (new_crtc_state->bigjoiner_slave)
+		return;
+
 	encoder = intel_get_crtc_new_encoder(state, new_crtc_state);
 	intel_mpllb_readout_hw_state(encoder, &mpllb_hw_state);