drm/bridge: analogix_dp: Fix timeout of video streamclk config

The STRM_VALID bit in register ANALOGIX_DP_SYS_CTL_3 may be unstable,
so we may hit the error log "Timeout of video streamclk ok" since
checked this unstable bit.
In fact, we can go continue and the streamclk is ok if we wait enough time,
it does no effect on display.
Let's change this error to warn.

Cc: Douglas Anderson <dianders@chromium.org>
Signed-off-by: zain wang <wzz@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423105003.9004-16-enric.balletbo@collabora.com