drm/omap: panel-dsi-cm: use DSI helpers

After converting the driver to mipi_dsi_device we can use the generic
message helpers to simplify the driver a lot.

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/20201215104657.802264-14-tomi.valkeinen@ti.com