dt-bindings: display: Start the info graphics with HS/VS change

The VS signal change is synchronized to HS signal change, start the
info graphics with that event, instead of having that event occur in
the middle of it.

Scope trace of DPI bus with HS/VS active HIGH looks as follows:
         ________________...__
VS...___/__         __        \______...
HS...___/  \_______/  \__...__/  \___...
        ^                        ^
	|                        |
        |    Used to start here -'
	|
	'--- Start info graphics here

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230221200407.16531-1-marex@denx.de