drm/bridge: tc358768: Calculate video start delay

Calculate video start delay based on the display timing instead
of hardcoding it to a default value. This fixes "trembling" display
output on Asus Transformer TF700T which uses Panasonic VVX10F004B00
display panel.

Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # Asus TF700T
Tested-by: Maxim Schwalm <maxim.schwalm@gmail.com> #TF700T
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211002233447.1105-4-digetx@gmail.com