OMAPDSS: panel-dpi: Add DT support

Add DT support for panel-dpi.

We disable the use of the backlight_gpio as it should be handled via
backlight framework with DT boots.

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