drm/drm_dp_mst:remove set but not used variable 'origlen'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/drm_dp_mst_topology.c:3693:16: warning: variable
‘origlen’ set but not used [-Wunused-but-set-variable]
  int replylen, origlen, curreply;

It looks like never use variable origlen after assign value to it.

Fixes: ad7f8a1f ("drm/helper: add Displayport multi-stream helper (v0.6)")
Signed-off-by: Bo YU <tsu.yubo@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200118080628.mxcx7bfwdas5m7un@kaowomen.cn