drm/bridge/synopsys: dsi: extend the prototype of mode_valid()

To evaluate the validity of a video mode, some additional internal
value has to be passed to the platform implementation.

Extend the prototype of mode_valid().

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@foss.st.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211218215055.212421-2-antonio.borneo@foss.st.com