drm/omap: panel-dpi: implement support for a vcc regulator

To allow supporting displays that need some logic to enable power to the
display try to get a vcc-supply property from the device tree and drive
the resulting regulator accordingly.

Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>