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