drm/omap: dsi: cleanup initial vc setup

As we now have a fixed setup for VCs (VC0 for video stream, VC1 for
commands), we can simplify the VC setup.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-80-tomi.valkeinen@ti.com