drm/omap: dsi: move structs & defines to dsi.h

Move structs and defines to a private dsi.h header file to make dsi.c a
bit easier to navigate. Also move the (now) private structs and defines
from omapdss.h to dsi.h.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-75-tomi.valkeinen@ti.com