drm/bridge: ti-sn65dsi86: Poll for DP PLL Lock

Instead of just waiting and hoping, actually poll for the pll lock to be
acquired. As a bonus, this should be significantly faster than the
sleep.

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/20180813213058.184821-6-sean@poorly.run