drm/omap: dsi: cleanup dispc channel usage

The "channel" usage in omap dsi driver is confusing. As the first step,
change "channel" to "dispc_channel" when dealing with the dispc 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/20201215104657.802264-65-tomi.valkeinen@ti.com