drm/mcde: dsi: Delay start of video stream generator

The initialization order for DSI video mode is important - if we
enable the video stream generator (VSG) before the MCDE DSI formatter
starts sending pixel data, it will immediately run into an error and
disable itself again.

Avoid this problem by delaying the activation of the VSG
until the MCDE DSI formatter is properly set up and running
(i.e. when mcde_dsi_bridge_enable() is called).

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106165835.2863-5-stephan@gerhold.net