drm/bridge: ti-sn65dsi86: Poll for training complete

Instead of just waiting 20ms for training to complete, actually poll the
status to ensure training is finished.

Changes in v3:
- Added to the set

Cc: Sandeep Panda <spanda@codeaurora.org>
Reviewed-by: Sandeep Panda <spanda@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180813213058.184821-7-sean@poorly.run