drm/omap: dsi: switch dsi_vc_send_long/short to mipi_dsi_msg

Simplify the DSI encoder by using mipi_dsi_msg for
dsi_vc_send_long and dsi_vc_send_short. Further improvements
require cleaning up the channel allocation code first.

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/20201215104657.802264-12-tomi.valkeinen@ti.com