OMAPDSS: register dss drivers in module init

We do the dss driver registration in a rather strange way: we have the
higher level omapdss driver, and we use that driver's probe function to
register the drivers for the rest of the dss devices.

There doesn't seem to be any reason for that, and additionally the
soon-to-be-merged patch "ARM: OMAP: omap_device: remove
omap_device_parent" will break omapdss initialization with the current
registration model.

This patch changes the registration for all drivers to happen at the
same place, in the init of the module.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>