OMAPDSS: DSI: fix DSI channel source initialization

During the initialization of the DSI protocol registers, we always set
the sources of all DSI channels to L4. However, we don't update the
value in the dsi_data, so we may end up with a different value in the
register and in the dsi_data, leading to DSI problems.

This patch fixes the issue by initializing also the channel source in
the dsi_data.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>