drm: renesas: shmobile: Use struct videomode in platform data

Replace the drm_mode_modeinfo field with videomode that includes more
signal polarity flags.  This simplifies driver handling of panel modes
and prepares for DT support.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[geert: Simplify]
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/4312e56de424d94399c6105e7159317eae86c9d5.1694767209.git.geert+renesas@glider.be