drm/bridge/tc358764: fix drm helper name

Recently drm_mode_connector_attach_encoder changed it's name. The change
was not noticed by bridge author, as a result gcc reports compile error
on next branch.

Fixes: f38b7cca ("drm/bridge: tc358764: Add DSI to LVDS bridge driver")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180727094859.27727-1-a.hajda@samsung.com