drm/omap: dsi: drop useless channel checks

A DSI peripheral can have virtual channel ID of 0-3. This should be
always the case, and there's no need in the driver to validate the
channel.

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/20201215104657.802264-64-tomi.valkeinen@ti.com