OMAPDSS: public omapdss_register_output()

In order to allow multiple display block in a video pipeline, we need to
give the drivers way to register themselves. For now we have
the omapdss_register_display() which is used to register panels, and
dss_register_output() which is used to register DSS encoders.

This patch makes dss_register_output() public (with the name of
omapdss_register_output), which can be used to register also external
encoders. The distinction between register_output and register_display
is that a "display" is an entity at the end of the videopipeline, and
"output" is something inside the pipeline.

The registration and naming will be made saner in the future, but the
current names and functions are kept to minimize changes during the dss
device model transition.

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