drm/omap: dsi: drop virtual channel logic

This drops the virtual channel logic. Afterwards DSI clients
request their channel number and get the virtual channel with
the same number or -EBUSY if already in use.

The change here is not strictly speaking correct, as it combines the VC
(DSI's "configuration block") and virtual channel ID (the ID sent in the
DSI packets). But as we currently only support a single DSI command mode
panel, this works fine: we always use VC0, and VC ID 0.

This needs more work to support video mode panels, but that can be done
after moving to DRM bridge and panel model, after which we can do that
work with the proper APIs.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-9-tomi.valkeinen@ti.com