OMAPDSS: DSI: remove omap_dss_device uses

The role of struct omap_dss_device will change in the future. The exact
details of that are still a bit unclear. However, the less uses of
omap_dss_device we have, the easier the change is in the future.

This patch removes uses of omap_dss_device from dsi.c, where it can be
done easily. Mostly this means passing dsi platform device to functions,
instead of the omap_dss_device.

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