drm/omap: dsi: use separate VCs for cmd and video

For command mode panels we can use a single VC for sending command and
video data, even if we have to change the data source for that VC when
going from command to video or vice versa.

However, with video mode panels we want to keep the pixel data VC
enabled, and use another VC for command data, and the commands will get
interleaved into the pixel data.

This patch makes the driver use VC0 for commands and VC1 for video.

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/20201215104657.802264-71-tomi.valkeinen@ti.com