drm/msm/dp: do not end dp link training until video is ready

Initialize both pre-emphasis and voltage swing level to 0 before
start link training and do not end link training until video is
ready to reduce the period between end of link training and video
start to meet Link Layer CTS requirement.  Some dongle main link
symbol may become unlocked again if host did not end link training
soon enough after completion of link training 2. Host have to re
train main link if loss of symbol locked detected before end link
training so that the coming video stream can be transmitted to sink
properly. This fixes Link Layer CTS cases 4.3.2.1, 4.3.2.2, 4.3.2.3
and 4.3.2.4.

Changes in v3:
-- merge retrain link if loss of symbol locked happen into this patch
-- replace dp_ctrl_loss_symbol_lock() with dp_ctrl_channel_eq_ok()

Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/1628196295-7382-7-git-send-email-khsieh@codeaurora.org
Signed-off-by: Rob Clark <robdclark@chromium.org>