OMAPDSS: panel-dvi: add PD gpio handling

The driver for the TFP410 chip should handle the power-down signal of
the chip, instead of the current way of handling it in the board files.

This patch adds power_down_gpio into the device's platform data, and
adds the necessary code in the driver to request and handle the GPIO.

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