disp: msm: sde: avoid null pointer dereference

This change avoids null pointer dereference in encoder kickoff.

Change-Id: I83c2c6f327ffb367a1cf5fc3a6cf0309a1187441
Signed-off-by: Narendra Muppalla <quic_nmuppall@quicinc.com>

msm/sde/sde_encoder.c

@@ -4658,6 +4658,10 @@ void sde_encoder_kickoff(struct drm_encoder *drm_enc, bool config_changed)
 
 	/* update txq for any output retire hw-fence (wb-path) */
 	sde_kms = sde_encoder_get_kms(&sde_enc->base);
+	if (!sde_kms) {
+		SDE_ERROR("invalid sde_kms\n");
+		return;
+	}
 	if (sde_enc->cur_master)
 		_sde_encoder_update_retire_txq(sde_enc->cur_master, sde_kms);