drm/dsi: Add to DocBook documentation

Integrate the MIPI DSI helpers into DocBook and clean up various
kerneldoc warnings. Also add a brief DOC section and clarify some
aspects of the mipi_dsi_host struct's .transfer() operation.

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