OMAPDSS: manage output-dssdev connection in output drivers

We currently attach an output to a dssdev in the initialization code for
dssdevices in display.c. This works, but doesn't quite make sense: an
output entity represents (surprisingly) an output of DSS, which is
managed by an output driver. The output driver also handles adding new
dssdev's for that particular output.

It makes more sense to make the output-dssdev connection in the output
driver. This is also in line with common display framework.

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