drm/omap: dsi: split video mode enable/disable into separate func

Clean up the code by separating video-mode enable/disable code into
functions of their own.

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/20201215104657.802264-81-tomi.valkeinen@ti.com