OMAPDSS: DPI: change regulator handling

Regulator handling for DPI and SDI is currently handled in the core.c,
using the 'virtual' omapdss platform device. Nowadays we have proper
devices for both DPI and SDI, and so we can handle the regulators inside
the respective drivers.

This patch moves the regulator handling for DPI into dpi.c.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>