drm/drm_dp_mst_topology: Remove set but never used variable 'len'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/drm_dp_mst_topology.c: In function ‘drm_dp_send_query_stream_enc_status’:
 drivers/gpu/drm/drm_dp_mst_topology.c:3263:6: warning: variable ‘len’ set but not used [-Wunused-but-set-variable]

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-4-lee.jones@linaro.org

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -3260,7 +3260,7 @@ int drm_dp_send_query_stream_enc_status(struct drm_dp_mst_topology_mgr *mgr,
 {
 	struct drm_dp_sideband_msg_tx *txmsg;
 	u8 nonce[7];
-	int len, ret;
+	int ret;
 
 	txmsg = kzalloc(sizeof(*txmsg), GFP_KERNEL);
 	if (!txmsg)
@@ -3281,7 +3281,7 @@ int drm_dp_send_query_stream_enc_status(struct drm_dp_mst_topology_mgr *mgr,
 	 */
 	txmsg->dst = mgr->mst_primary;
 
-	len = build_query_stream_enc_status(txmsg, port->vcpi.vcpi, nonce);
+	build_query_stream_enc_status(txmsg, port->vcpi.vcpi, nonce);
 
 	drm_dp_queue_down_tx(mgr, txmsg);