OMAPDSS: remove DT hacks for regulators

For booting Panda and 4430SDP with DT, while DSS did not support DT, we
had to had small hacks in the omapdss driver to get the regulators. With
DT now supported in DSS, we can remove those hacks.

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