OMAPDSS: remove dssdev uses in trivial cases

In the future the "dssdev" parameter passed to output drivers will
change its meaning. Instead of being a pointer to the panel device, it's
a pointer to the output instance.

To make the transition easier, some of the uses for this dssdev
parameter can be easily removed.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>