drm/dsi: Constify mipi_dsi_msg

struct mipi_dsi_msg is a read-only structure, drivers should never need
to modify it. Make this explicit by making all references to the struct
const.

Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>

include/drm/drm_mipi_dsi.h

@@ -77,7 +77,7 @@ struct mipi_dsi_host_ops {
 	int (*detach)(struct mipi_dsi_host *host,
 		      struct mipi_dsi_device *dsi);
 	ssize_t (*transfer)(struct mipi_dsi_host *host,
-			    struct mipi_dsi_msg *msg);
+			    const struct mipi_dsi_msg *msg);
 };
 
 /**