OMAPDSS: DSI: set regulator voltage to 1.8V

Set the DSI vdd regulator voltage to the required 1.8V.

This is required for the case when the regulator in the DT data defines
a range of allowed voltages. In this case it's required to set the
voltage, as otherwise enabling the voltage fails.

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