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>