OMAPDSS: simplify submodule reg/unreg code

Now that we are using components in omapdss, there's no need for
separate handling of dss and dispc driver init. Thus we can move the dss
and dispc init and unit func pointers to the lists we use for the other
dss submodules.

We can now also handle errors returned by the registration functions
properly: if registering a driver fails, we can stop processing and
return the error.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>