OMAPDSS: SDI: fix regulators for DT

SDI requires a regulator to operate. This regulator is, for some reason,
currently attached to the virtual omapdss device, instead of the SDI
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 SDI 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 SDI.

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