drm/omap: dsi: untangle vc & channel

DSI virtual channel and hardware VC blocks have gotten tangled as
described in the previous commits. This has not caused any issues, as
the value for both is 0, so it happens to work.

To fix the issue, change the code to use the correct one of the two.

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/20201215104657.802264-68-tomi.valkeinen@ti.com