OMAPDSS: DPI: fix regulators for DT

On some platforms DPI requires a regulator to be enabled to power up the
output pins. This regulator is, for some reason, currently attached to
the virtual omapdss device, instead of the DPI device. This does not
work for DT, as the regulator mappings need to be described in the DT
data, and the virtual omapdss device is not present there.

Fix the issue by acquiring the regulator in the DPI device. To retain
compatibility with the current board files, the old method of getting
the regulator is kept. The old method can be removed when the board
files have been changed to pass the regulator to DPI.

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